I would go electric simply because it is hassle free making it easy and quick to take out to a field and fly. Simplicity, ease of use, and stability is key imo for a trainer. Nothing nicer than an electric to meet those needs and later on with experience you can go to a nitro based plane and gain the knowledge/experience to tune/maintain a nitro motor.
With a trainer you need to learn to take off, track/align with designated runway, land, and be able to recover from any mistake/wind. Also should join a club and must fly with an instructor to save money and time.
I learned on a glider and I love gliders. IMO they are far easier to fly since they can slice through the wind and are very stable. But many gliders do not have landing gear (in the 1100-1600mm wingspan range) however you need to slow down as much as possible for a belly landing.
I just flew my first landing gear equipped plane this past weekend and found it to be rather easy. However because I have flown alone and learned alone I never learned tracking and sharing air space comfortably with other pilots. Saturday was my first attempt with lots of pilots present and I was too nervous. I was nervous about everyone around me and it really impacted my piloting skills to the point where I was jittery, and over correcting my plane, and luckily I had a forum member/club member present to advise me and i had a fairly smooth first landing in the grass since i couldnt track with runway that well... I came back to the field on sunday with no one around and was able to get an hours worth of take off, tracking, and landing back to bad (i think over 40 landings). I found my flight performance to be so controlled, relaxed, smooth, and landings to be far easier with no one else flying. For this reason I highly suggest you learn to fly at a club. It makes it a lot easier, right now i'm having some difficulty adjusting but I'm getting better.
If you want a glider then may I suggest (i know i've been recommending this a lot) the AXN Clouds Fly. Pretty cheap and with all the electronics you need it is under a 100 bucks. But it has no landing gear. It is stable for first time flier and a larger prop can provide some speed later on.
http://www.hobbyking.com/hobbyking/s...arehouse_.html
put a 30amp esc in it and some servos and you are good to go!
It is extremely durable. I've crashed it numerous times and it still works and flies well. If you break a wing on this plane then a balsa plane would have been demolished.
But I am noticing almost all of the planes are sold out at hobbyking... I guess its the season/good weather.
As for a plane with landing gear this one is pretty nice too.
http://www.hobbyking.com/hobbyking/s...er_Balsa_.html
I'm going to buy one when in stock to help a friend learn to fly.
this is just my opinion, there are many good type of trainers both based on airplane design and power system. My opinion is pretty much from what I experienced as a solo pilot in training.
Also a nexstar EP plane is nice too but a little pricier, they also have a nitro version too. And As other have stated if you go/join a club you may be able to score some deals from newer pilots that are looking to move up from a trainer. I suggest you look into that first before buying anything online.