If the little bit of extra money isn't an issue for the 60 size trainer, go for the 60. It'll fly smoother and better. The 61 four stroke would be plenty for it, even as would a 46 two stroke. You'd really enjoy it with the 60 four stroke. The trainer will still fly great with either. I had a 60 two stroke on my 60 size hobbico trainer (same as tower) years ago, and never needed to fly above half throttle. Trainers don't need half as much power as a heavy warbird.