Base it on your leg length. If you have lengthy legs, go with the large. If you are a little stalkier, get the medium.

You do want the bike to fit a touch small if you are actually going to be freeriding Banshee also fits big. The tt's are nice a long. I can ride my gf's small and I am 6'5. It isn't all the comfortable but it is doable. I could likely live with a medium