Well, it's not the first, but it will be different, so here goes... I was a long time user of Realflight G2... Back before I even knew any other products existed. The G2 served it's purpose, and worked fairly well, though the 3D physics were off a bit. However nowadays... Well, just can't touch either of these two big boys. It wouldn't be fair to not mention G3. I don't personally nor have I ever owned G3. I HAVE however used it at a friends house for a considerable amount of time. I will not discuss this product in this comparison because in my opinion there is none. Perhaps it has gotten better with recent updates, but when I initially tried it (back around christmas time) it was pretty horrible. The planes had an "off" feel to them. Not sure exactly how to describe, but they just didn't "feel" real... Not to mention the many bugs that I experienced myself (invisable trees, etc...) Anyway, on to the core of our subject...

In case your wondering why I took time to do this... It's really quite simple. When I was first looking at getting a new sim I spent a lot of time researching trying to decide which one to buy... The reason, or rather the HOPE was that I could avoid doing just exactly what I did.. Buying two sims... It's really a pain to try to decide, and neither of these sims is cheap by any means, so my ultimate goal in this is to help others like myself who may be looking for specific things to have one reference point to refer to. Hopefully this will save someone a little bit of money and give me the satisfaction of knowing I may have helped a few of my fellow modelers out.

To make this a little different I'll break it down into areas....

3D Airplane Flight Physics:
AFPD: The 3D physics are right on. The shockflyers feel exact, the OMP models are great, and overall it is VERY realistic.
XTR: Again the 3D physics are very good. Maybe just a "tinge" under the AFPD. Most people would probably never notice the difference.
Notes about both: Wolfgang (developer of XTR) seems almost religious about getting it "right".. You tell him WHY you think a certain plane is not "acting" right, and he'll fix it... I haven't tried this with IPACS, but from what I've seen on the forums (and one personal opinion of mine) this just wouldn't happen. However there do seem to be a few "non-IPACS" people out there that are more than willing to help people with setup of planes, etc...

Standard Airplane Flight Physics:
AFPD: Almost right on the nose. My only complaint seems to match what a few others have said. The planes sometimes feel just a SLIGHT bit "floaty". On several occasions I've ended up going into the aircraft editor and ADDING weight to planes to get them to feel a little more realistic.
XTR: Seem almost dead on to me. I have seen some posts that say it seems a little "too snappy"... In my own opinion, flying planes that I have flown in real life, they seem almost exact on to me. Keep in mind that I also tend to like to keep the CG just a tad bit behind the "suggested" spot. If you were flying the same model with the CG moved forward a tad, chances are XTR would feel a little more "lively" to you.
Notes about both: Rather your a seasoned pilot or are hoping to teach yourself to fly, either of these sims will be fantastic. That leads to our next area...

Heli physics (3D and standard)
AFPD: I hate to say it, but it's true... The helis in AFPD that come out of the box are CRAP. They don't fly worth a darn.
XTR: The XTR helis are AWESOME right out of the box. The ONLY tweaking you may have to do is dropping the "linkage play" and "hover instability" a bit. They actually make it a little to squirmy. This would be great though if you wanted to learn to fly micros.
Notes about both: If your an experienced pilot, you can spend a lot of time tweaking AFPD helis to get them to feel realistic. With a LOT of tweaking you can get a heli that flies and feels VERY realistic. For a beginner though, or someone looking to teach themselves to fly heli this isn't an option. In my honest opinion I don't think ANYONE at IPACS actually flies a heli... If they did, they would never have allowed the sim to go out the door with those default settings. I spend two days getting the raptor to feel like a real one... Once it's tweaked, it's very close, but XTR is almost perfect right out of the box.

Heli settings
Notes about both: I have to mention one other point here with regards to the helis in these sims. As anyone who flies heli's well knows, the FIRST AND FOREMOST thing you have to do once the ship is set up, is set up your radio! This involves among other things, dialing in your pitch curve followed by your throttle curve. With XTR this is a cinch. You start the program in window mode and turn on the "data display" option. Kill the motor on the heli (yes, the engine actually dies if you idle it too low.. The clutch works perfectly as well... ) and in the status bar of the window it will display the current collective pitch. Using this you can easily set your pitch curve just like you would on the real thing with a pitch gauge. Once your pitch curve is set for all your normal and idle-up modes, you can then move on to throttle curve setup. Here you just start the heli back up (reset) and adjust the curve for your various stick positions while watching the indicated RPM of the head... The pitch settings as well as the RPM settings are VERY realistic.. If it says the head is turning 1650 RPM, it FEELS like it's turning 1650 RPM... AFPD fails in this area. Again it feels like noone at IPACS knows much about helis... There is a "data display" option, but it ONLY displays RPM... This makes setting your pitch curves almost a guessing game... Yes, by setting the max and min pitch values, you SHOULD be able to mathmatically calculate all the percentages that are required to get various pitches at various percentages of throw... Ever tried that with a real heli? Might get close but.... well... I've tried this myself in the sim and it IS close, but you can tell it's just not right. Also the RPM display for some reason seems a little "off" Actually it seems off by just about exactly 200 RPM... If it says it's turning 1450 RPM, it feels like it's turning 1650... .If it's turning 1650, it feels like it's turning 1850... I've tried posting info about this in the "Aerofly Pro Deluxe" support forum, (about the pitch display and the RPM display) but my posts were deleted... Supposedly because they didn't follow the rules of the forum... Apparently it can ONLY be used for support issues like "my sound card isn't working", not "I need these data displays to be able to program my radio correctly and be able to actually USE your program".... (sorry, that part frustrated me a bit) Anyway, I posted the same comment on IPACS forum, but the response was that it's apparently on a "TODO" list, but they haven't decided yet rather to implement it with a patch or a version upgrade....

Using as a tool to learn to fly:
AFPD: If you learn to fly in AFPD and do it well, you'll have no problems at the field. Simulation speed adjustment.
XTR: Same here, if you can fly, you'll fly at the field as well. Hovering and Torque rolling training.
Notes about both: Reflex seems just a little more straight forward regarding setup in some areas. If your a beginner that may help, however, if you get bored easy, Aerofly has a couple of "neat" features that may break of the boredom occasionaly (ballon popping, pylon racing, etc...) One other thing to keep in mind, Reflex does have some nice "training" features. First is the "no fly zone" some people complain about this.. Basically it's an area directly OVER the pits and spectators... If you fly over this area the screen is filled with red slashes (to let you know it's a no fly zone) and then your plane is placed back on the runway in a take off position. I think it's a great idea myself, especially for a beginner.. If you go to a real field and fly over the pits your gonna get your arse kicked to the curb... If your gonna learn, learn right... It's not that you can't do this with AFPD as well... XTR just forces the issue. AFPD has a simulation speed adjustment. This allows you to S L O W the sim way down, try manuevers in slow motion, then gently crank up the speed... This can greatly help you when trying to perfect you manuevers. XTR does not have that feature. XTR offers a "hover" training for helis... Basically you can "turn on or off" certain controls... For example, you say you ONLY want to worry about your fore/aft cyclic movement... The heli sits in a perfect hover EXCEPT for fore/aft cyclic... Your responsible for controlling that... Later you can turn on JUST left/right cyclic... Then JUST throttle.... Then JUST tail rotor.... Then start combining them... Maybe do JUST cyclic... then JUST throttle/tail rotor... Then you can bring everything together and you'll be hovering like a mad man. Same with torque rolling... You tell it WHAT specific parts of the plane you want to control.. Elevator? Rudder? Both? Throttle? Aileron? Etc...

One note that needs to be stuck in at this point... A Sim is a GREAT tool for learning to fly, but it is NOT a replacement for an instructor. There are MANY things that a sim can teach you, but there are also MANY things it can not! ALWAYS get an instructor to look over your plane BEFORE you attempt to fly it, make sure you join the AMA and a local club. NEVER attempt to fly on your own...

Graphics:
AFPD: The graphics are AWESOME. The photo-realistic fields are .... photo-realistic.. Everything is mapped very well (except one field (sparling) that was apparently never finished but is supposed to be fixed sometime in the future (the fence isn't mapped out right). The 3D scenes are very well done as well.
XTR: The graphics again are AWESOME. The scenes (photo-realistic) are the same quality as AFPD. The models themselves are slightly better than AFPD. They look a little more realistic in my opinion.. Especially the helis. The 3D scene (there's only one in the box) for XTR sucks... It's on par with G2 quality. Not great at all.
Notes about both: Either of these is going to be a real close second for actually packing it up and heading to the field. It's unbelievable how "involved" you can get when you "feel" like it's real.. I've on many occasions ACTUALLY had that "sinking" feeling in my stomach as I was practicing a manuever and realized I wasn't going to get out in time on BOTH of these sims... It actually took a second to realize, HEY, it's just a sim, no fixing required! The G2 and G3 sims just can't compare, at least not with the photo-realistic scenes... I've heard some people say they don't like the PR scenes.. I'm not really sure why... Perhaps they were used to older sims where you couldn't "interact" with objects? I just can't imagine anyone not liking them myself. It really is just THAT real...

Compatibility:
AFPD: The home page recommends a pretty healthy setup. I'm running a GeForce 5700LE which is a decent card, but not top notch by any means... On the PR scenes I usually average something like 170+ frames per second. The 3D scenes slow down a bit (down to around 60+ frames per second) This is still MORE than adaqute for the sim. If you plan on flying mostly in the photo-realistic scenes you can probably get away with very low hardware specs. AFPD does apparently have some issues with sound as well. Several people have reported problems with not hearing engine sounds. This apparently comes from those who have surround sound. You HAVE to disable the surround sound and put in in normal stereo "2 speaker" mode to get it to work. There seems to be another issue some have (including myself) which is with the engine sound being "staticy" It's there, and you can hear it, but it sounds noisy and un-realistic. I had an SB Live card, and even tried removing it and using the built in sound on the motherboard, but both had the same result. Not sure what this is a problem with. It's not a huge deal, but it does detract a bit from the realism if you happen to have this problem.
XTR: The requirements seem about the same as for AFPD's photo-realistic. My frame rates are just slightly less (around 150+) with XTR. Perhaps some extra processing, or the fact that the models are much more accurate. Not sure.
Notes about both: I think any reasonably newer computer is going to run either of these sims with no problems. The only REAL requirement they both have is that you support DirectX9... AFPD does also require OpenGL I believe.

Selection of Aircraft:
AFPD: AFPD comes with a ton of planes, and even more are available on the web. My ONLY complaint about them is the TYPES of planes that are available. I'm not sure exactly why this is, but there doesn't seem to be many models that I'm that familer with. For example, I fly a p51-d, Ultra Sport 40+, Twist 3D, Fun-Air, and a Raptor 30. The stock raptor obviously wasn't great (see above section on helis), and the p51-d that comes stock is a BIG model. Didn't really compare to my 60 size plane. I guess perhaps this is because most of the people developing the models are over seas? I dunno, but I just couldn't seem to find the planes I was looking for... Keep in mind though there are TONS of them out there to choose from. Maybe I'm just picky.. You also have to be careful as some models that were designed for Aerofly will not work correctly with pro deluxe. Many are being converted though. Also I have to mention the OMP planes now. LOTS of OMP models are available, and while I don't own a real one, everyone says they fly VERY realistically.
XTR: There are LOTS of airplanes and helis to choose from out of the box as well... Many seem to be a little more familer to me (Joker, Logo, Corona, Raptor 30, 50 and caliber 30 to name a few as far as helis.... Katana Eagles, P-40, Flip 3D, Pitts Special, and Tiger Moth to name a few planes...) There seem to be more downloads available for XTR as well, This is probably due to the longer period of time XTR has been available. More models will probably start to appear for AFPD as time goes on.

Radio Setup:
Notes about both: There really isn't much to say in this area. EXCEPT... AFPD is a little better in my opinion about one thing... It does use seperate (or rather CAN) for the ailerons... This allows you to use mixes in your radio for spoilerons, flaperons, etc... XTR does NOT support this... It's a single aileron channel... There IS however support for flaps in XTR.... It's just a different channel. AFPD also comes default in mode 1... So us US guys will have to make some changes... Reflex doesn't come set for anything, you assign channels right from the start.

Price I paid for NEW:
AFPD: $170 (free shipping) from OMP (www.aeroflypro.com)
XTR: $185 (free shipping) from Xtreme RC (store.xtremercproducts.com)

Summary:
Here's the bottom line...

If your just looking for something to use on those rainy days... Your going to be thrilled with EITHER of these sims... Neither one is going to let you down in any way. They are both a blast...
If your a beginner, and want to learn to fly, either will do a fantastic job, but XTR may have a slight edge because of it's no fly zone.
If your a 3D pilot, AFPD is probably the way to go. It has a lot of stock 3D planes, and again the 3D physics are very accurate
If your a foamie pilot, AFPD is DEFINATELY the way to go, the foamies in AFPD are dead on.
If your a heli pilot, don't think twice.. Buy XTR
If your a pattern pilot, AFPD probably has a bit of an edge now
If your a sport pilot, your probably going to be equally happy with either
If your a turbine or jet pilot, don't think twice.. Buy AFPD

I hope at least SOMEONE out there finds this information useful... If you have any specific questions about EITHER product please feel free to contact me and I'll do my best to answer.