Perhaps I'm a terrible person? But I have a very strong allegiance to those who have helped me at all/made NAPs with me etc...and a very strong aggressiveness toward those that have attacked me/hurt me in any way. I do a fairly decent job of keeping it within the bounds of the game, but, in this case, I would always help whichever side helped me more/attacked me less.
On the other hand, just do whatever is most challenging to see if you can. I mean, if the less-powerful nation can probably win with your help, then try to help in such a way that you become a lot stronger as well, and then try to beat that nation, too!
