I like to select my power systems depending on what type of flying I like.

For 3D, I use at least 150 watts per pound. For aerobatic flight, I'll use 100 watts per pound. For scale flight, and for planes with particularly big wings (like a GWS Slow Stick), I'll use 50 watts per pound.

Generally, for a trainer, I recommend at least 100 watts of power, so that you can have enough power to get out of trouble.