Most Helpful Guy
Definitely tell one of them as soon as possible. The longer you drag it on the worst it will be for you and the guy you aren't choosing. Especially if they both like you. Do both of you a favor and tell him right away. Just be honest. It sucks and I know you don't want to hurt either one, but you are hurting the guy you're not choosing more by not telling him and letting him think he has you.
Most Helpful Girl
this is a tough one. You should just tell the guy that you're not interested. You can still be friends if he wants. If he likes you he should understand. So I just say. Tell him directly. And Do it in person or at least over the phone. Sorry you have to do it in the first place that's no fun.