I think its all a matter of preference. Unless your all XC, I like a smaller frame. I ride a 56cm on my fixed roadie, and 16" on my MTB. But then all brands will fit differently, even at the "same" size, due to small geo differences. I'd say go to a LBS and get fitted if you can, and test ride different sizes of different brands. And if you want to do more XC, you could go with a larger frame, but for DJ or DS you may want something smaller.