With a computer you get higher resolutions and more accuracy, but its also more expensive since you need to have a high end pc/vid card to enjoy the high settings.
I must say I bought the game today on pc - still getting used to it but it's incredible... just doing missions at the moment but the graphics are awesome.
go with a PC Version, because you already bought Internet for it unless you already have Xbox Live.. but I say its easier to control on PC, but easier to get Headshots on Xbox