PC: user created mods, unofficial patches and other adjustments to the game are all things you'll miss out if you play the Xbox version. Also, as stated earlier, the game's probably easier to handle with mouse and keyboard than with a gamepad, and even if it is easier with a gamepad, you can always connect one to your pc :p.
The only reason i'd buy a console version instead of a pc version is if I'm afraid my PC won't meet the requirements. In all other cases: PC version>console version

[Linked Image from miniprofile.xfire.com]