PC all the way. The 360 version was an excellent attempt, but failed in many areas as it was hampered by the hardware -- it's major issues are primarily the controls and the graphics (which have texture issues all over the place, and often lag). The PC resolves almost all these issues (but still has a crappy inventory system), and the game controls far, far better with keyboard and mouse (although, if you want to hamper yourself, you can still use a gamepad on PC -- best of both worlds, I suppose).
Really, the 360 version looks terrible when running beside a 1080p PC version, which is, of course no surprise.
Sure, the game plays on 360, but it's a horrible experience, by comparison. And, yes, I've played both (own the PC version and completed it 2 times; played and completed the 360 version at a mate's house when baby-sitting, which was a bit of an eye-opener as it was far poorer than I expected, but I got him some extra achievements, and it was still fun enough).
Oh, and the first DLC is free on PC (I've not bothered to check out the second one, as the first was so uninspiring).