Long story short: Dvorak is better than Qwerty by a long way, Colemak is better still, but not by as much. If you know Qwerty and were thinking of learning Dvorak, learn Colemak instead. If you know Dvorak already, don't worry about Colemak - it'd be a lot of effort for not much benefit.

(Cue Dvorak devotees with burning stakes and pitchforks?)