I highly recommend buying it on PC as opposed to the Xbox, the requirements don't ask for too much so any half-decent computer should run it pretty smoothly.
As with most valve games on PC the community is still very much populated. Global Offensive also is quite a bit easier to get into for new-players, with casual game-modes and the like.
So get it on PC. It will most likely be cheaper on Steam anyways at $15.00.
Plus mods and zombie gamemodes.