wired. im sure some of it depends on quality of the mouse, but generally speaking for gaming wired is better. and weight, you need batteries in a wireless, can make a mouse pretty heavy. i think some of it is personal preference too.
if it's gonna be for gaming, i would suggest wired... there's nothing like having your batteries run out during a heated multiplayer game... it really sucks... plus having to buy batteries gets costly...
wired i reckon, cos there's nothing worse than having full ammo but only to find your legs don't move on the battlefield due to battery failiure arggggghh