Ah yes so many engines! Assuming your starting out, your more than likely going to want to look at the .40 or .46 size engines for a trainer.
(That is assuming your going to start out with a .40 size trainer of course! ) It's hard to go wrong here and to be honest, if its brand new one no matter which brand ya get its going to be of quality and meet your needs for a .40 size plane. I started with an OS .40 LA on my Hobbico Superstar and later strappedon a .46 FX for more power. But what ever brand you choose, good luck!