Personaly i dont think it matters, the big influence is cost, if you are happy spending twice as much money on a 4 stroke go for it. BUT i would recomend getting a 4 stroke that is as big as possible for the trainer, if the recomended 4 stroke is 53 - 60 go for the 60 as it will be more usable on your next plane, going this way may also give you balance problems so double check the C/G is correct