Recently (in the past 10 years), mountain bikes have been modifying the wheel size they come with. In general terms, the 26" wheels are stronger, lighter, and better for aggressive riding. The 29" wheels roll over bumps better and feel faster when riding. The 650b (27.5") splits the difference. 650b is a pretty new trend, so you'll pay a lot more for those. I ride 26" wheels because I'm small, old-fashioned, and because I love jumping off every rock and root I can find (tougher to do on a bigger wheel).