Controls:
x360 is limited to gamepad, PC has both keyboard and gamepad. PC wins.
Graphics:
x360 runs at 1024x600 stretched up to whatever your resolution is with no AA/AF, PC run everything up to 2560x1600 natively with 16xAA/16xAF, DoF, MB, HDR etc. PC wins.
DLC:
x360 gets it later, or sometimes never and costs money, PC gets it faster and for free. PC wins.
Mods:
x360 has none, PC gets them for free and you can make them yourself. PC wins.
Price:
x360 costs the same always, PC has a steam sale every other week. PC wins.
Patches:
x360 gets them late or never, occasionally with a price tag, PC gets them on time and for free. PC wins.
Playstyle:
x360 is limited to couch, PC can also do that when played with a gamepad or just wireless KB+M and can also be played at a desk or on a laptop. PC wins.
I think that covers it.
As for what game, there's a good bundle containing both on steam and I'm expecting a sale for it this month.