Well, I suppose I'm going to be the odd ball here and vote for speakers. Really, it all depends on the situation, though. For the normal PC gamer, headphones are going to (for the most part) WAY outperform any comparable solution.
I'm a somewhat special case in this instance, though, as my PC is hooked into my HT room. Of course I still have my trusty set of cans (Beyerdynamic DT-770s) if ever I need them, but to be honest, I don't think I would willingly go back to using them as my default listening device. Headphones just can't compares to the BIG sound you can get from speakers and a dedicated powered sub.
Well, it's just different, really. Each is good in it's own respect, but I would certainly feel pretty silly playing on a 110" projection screen with headphones on. :biggrin: