What do you need to do?
You're a young girl .. enjoy having fun with lots of people. Don't get serious about anyone.
Well do you like him? If not then your decision is rather obvious.
.....Alright. I've kind of been in the friend likes me, I like his friend thing. Avoid rejecting his friend, or telling him that you have feelings for his friend. It might evolve into the terrible "if I can't have you, nobody can" attitude. Try to maybe get to know his friend better? It's hard to tell at this point.
I get the feeling you will wind up with both of them for a little bit, then you will keep moving on to yet another guy.
have sex with his friend.
^ and all the skanky girls will mention you after they see you on mtv. after both of this advice - instant celebrity status