Definitely pick a major because it interests you, rather than because you think it will help on the MCAT. If you dislike the stuff you're studying, you'll procrastinate and do poorly. (At least, that's me.) And if you want extra coursework for the MCAT, you can usually always take the random other classes like biochemistry, anatomy, advanced physics, etc.
I'm double-majoring in biochemistry and comparative literature, and yes, I feel that those fields really helped me on the MCAT. But you have to study hard no matter what. Also, it's nice having two completely different majors, as it keeps my options open somewhat. My CPLT advisor was trying to get me to apply to complit MA programs as a backup in case I wanted to take a couple years off before med school to write. (Tempting!)