All mp3, be a MAN ABOUT IT! I'm doing all mp3 and my project is gonna be closer in length to Alter, but it's gonna sound beautiful!
I just feel that midi is VERY restricting when it comes to trying to build an atmosphere, whereas the higher file size allows more versatility for you, the creator. It doesn't have to sound like "obligatory video game", it can sound like the most epic film you've ever seen! And so many genres to choose from in mp3, midi almost feels like it IS its own genre.
Especially the battle music. I find myself hating midi battle music, at least when all the other music is higher quality, then the one thing out of the soundtrack you're forced to listen to over and over is the same midi. It just feels tired.
Anyway, dat's my 2 cents. The amount of extra space the game takes up is small compared to the improvements an mp3 score can make. Besides, 100-200MB? Still fits on a standard CD-R. No worries!