High quality stereo headphones:
The good: Great audio quality and clarity, music will sound great, sounds will be sharp and defined, etc.
The bad: No real positional audio, and no post-processing will give you true surround sound
The ugly: Sets you back a pretty penny
5.1 Gaming headphones:
The good: Varying degrees of surround capability, generally do very well with spatial positioning and adds a distinct advantage in FPS games
The bad: Sound quality is generally mediocre and definitely won't blow you away
The ugly: Some sets are constructed cheaply and will either have unbalanced sound or require software adjustment to be passably equal in volume across the channels
I'd say get the good stereo headphones and stay away from the 5.1 gaming headphones, but if you're a heavy FPS player the 5.1s will benefit you more - and audio quality isn't really an issue with games.