Eeeek, a split heart. Tough one.
It's hard to say too much without knowing you too well, but the best thing you can do is just try and work out who you prefer by yourself. You don't know for sure that if you pick one, you will lose the other, you may still remain friends.
I would say look at the logic/distance/frequency of seeing them, but when your heart comes into it, none of that makes a difference.
Maybe take a breather from both of them, and take some time by yourself to figure it out. If you want one as more than a friend, you will have to sacrifice the other and keep them as a friend.
If he's a good friend, he'll understand, he'll be hurt, but he'll get over it soon enough.
