If I were you, I would go with a .52. You can power back with that one, but with a .40, that is all you have. It will be slow, and underpowered with a .40. You also have the problem that with the 40, you will have an engine that will not work on most of the 40 size planes out there. if you go with the .52, you will have an engine that will work with just about any of them.