I have spent about 40 horus on each sim now, this analysis only includes PLANES, not helis, Jets, or Sail planes because I don't fly them nor can I get an opnion from anyone who has flown them.
The only thing I will talk abuot is physics becauase graphics and sceneries and 3d background, music and all that other junk is something I could careless about.
Also, I will only cover 3-d Flying, simply because all other types of flying in circles is just something you can practice on the field 300 feed in the air, you don't need a simulator if you are planning on flying around in circles and doing lopps. For that their is FMS and a ton of other free sims. SIMS are for learning 3-D


AFPD:
After playing around with AFPD, it let me do, pretty much every trick in the book for 3-D. And it puts the plane where it ought to be, e.g. after a blender, knife edge e.t.c. the plane sets it self to how it does in real life. The other thing with AFPD is that you don't have to go to extremes to get a plane setup for 3-D, e.g. you an set it up to do hovers without loosing the ability to do stuff like blenders and harier rolls. Out of the box plane sets up are really good. I do like having that little window on the side where I can see what position my plane is in, afpd has all these little toys that are fun to play with but like I said in the begining I'm not going to get into that.

Now here are the problems:
Gravity, My God, these planes devy gravity when in stalled positions, the simulator will SLOW everything down for you when you are in 3-D. I don't know what is going on but all of a sudden many of the stall points just go away, untill your plane starts generating lift again and then all of a sudden its back to normal flying. Now I realize thier is a simulation speed bar... now what the heck, I think 100% should be real life speed! But people say turn it up to 124%. Ok, did that but guess what now? You should see my planes do rolls, lol they spin like tires on a race car. So while ya, the planes act more to life at 124% speed while in 3-d, but that messes up a lot of patern stuff, rolls harrier rols all seems fake. Basically you give up 1 thing to do the other more realistically.

Other then that, I don't have any main problem with AFPD's physics engine is just that the response, gravity, and some other issues are out of wack, but the plane's behaviour is right on, doing 3-D is right the planes act like how do they do in real-life. You can tweak the planes out and you don't have to go to extremes to get it to do 1 thing while it looses the other. The "Environmental" physics are just off and it effects every other plane.


Reflex XTR:
I have been playing around with XTR for a bit now as well. Out of the box, man reflex seems great, gravity, tip-stalls, all the stuff that gets you on your toes and stops your heart when flying a real plane are simulated perfectly. The simulation is awesome for everything... BUT 3-D[&o]... basically for the good: Reflex is better at everything except how the plane handles in stalled positions.

Problems:
I'm going to jump striaght into the problems, and they are when a plane is in stalled conditions, or 3-D. You just can't get reflex to be happy with a plane. The stock planes are very hard to do 3-d, you need to to be on your finger tips the whole time, some would say its easier to do 3-d in real life then on the simulator. While ya, reflex isn't cutting you any slack nor slowing anything down like afpd does it just doens't put planes in positions that are in when doing 3-D stuff. In the stock planes, (Katana) is the only one you get and it set up pretty close to the real thing for everything else e.g. patern flying manuvers, knife edge e.t.c. but the control surfaces are only at 20, you will need to go to settings and change them. THe user made planes I downloaded, were obviously setup to do 3-d a litte "Too well" e.g. the extra 300s.. they were doing stuff that you CAN"T do, e.g. super responsive elevator, I would wiggle my plane back and forth like a worm on a hook. When I toned down the physics, e.g. lowered angle of attack and stall angles, bam my plane doens't want to snap rolls any more, or does the roll part but is obvioulsy not "snapping"...

So here is the deal with Reflex xtr in 3-d: Expect to spend a LOT of time getting your planes setup, I mean you have to tune the darn thing to the dime. Unlike AFPD, you can get your plane to ridicoulous stuff so don't mess around with physics like crazy, change one thing, test it, save (btw thier is a bug in reflex, you need to save the plane everytime you make a change and reload it for some things to to take effect). The only planes that I have found fly really darn close are: the Funtana with modfied settings by: Not 24, the Included Katana's with modfied settings by myself. Remember thier are about 3x as many settings in Reflex XTR as they are in AFPD, they are also harder to figure out because their is no GUI and it only gives you stuff in METRIC units not english.

But when you get a plane setup JUST right. Man that thing is awesome, HOWEVER..LOL your gonna hate me for this, the darn thing flies very close to how it flies AEROFLY and it seems that once again gravity is gone, tip stalls are gone...lol basically you just set up your plane like a plane would fly in AFPD.[:@][:'(] But then again your plane does everything fine and you are no longer playing a chess game with the physics engine.


CONCLUSION:
Reflex XTR is better in everything else except in 3-d when compared to AFPD. When a plane setup to do 3-D in reflex, it flies very close, or almost the same as it does in AFPD. So if 3-D is your main thing, AFPD will probably let you fast forward through all the config and pain and sweat you have to go through with Reflex. However overall, REFLEX is the better simulator if you have the time because it does all the other stuff better then AFPD e.g. pattern flying, warbirds, helis and does 3-d Just as wll as AFPD once you spend enough time setting your plane up e.t.c.

In the end, I have both SIMULATORS: AFPD & XTR. I like AFPD because thier are more downloads coming in for it then reflex, and I don't have to spend as much time to set my planes up and the stock planes fly really good for 3-D. I like Reflex because all other types of flying e.g. Warbirds which is my other passion is just so much more realistic in Reflex, simply because your plane doens't go into that "3-D" or stall position.

I generally belive unless you want to learn 3-D, you dont' need to buy a simulator. Circle flying, loops, rolls and inverted flight are something even a newbie can learn after 2 hours behind the controll stick at the field. 3-D flying is something you need a simulator for because this is when you need to have mastered all 4 of your control surfaces (yess throttle is a control surface in 3-d flying). If you still insist you want a simulator for generall flying Reflex would be the one to choose. For 3-d, I would probably go with AFPD because I would have less headaches setting planes up