Oh Good God, not this again. There have been huge flame wars on the subject.
I hope I can answer it and the nothing more will have to be said;
If you have a high-end computer, I'm talking a 7900GT or higher videocard, then you're better off.
If you have anything lower than a 7900GT videocard, then the Xbox 360 might be something you want to consider.
You can build a $1500 gaming PC and use a keyboard, or you can buy a $360 Console and use a controller. Because the fact is; the X360 will provide the same graphics as the 7900GT. You can plug the Xbox 360 into your PC LCD monitor, so please, no one say anything about the HDTV crap.
EDIT: Yea, the pics Frank Jaeger posted are the ones I used for my analysis also. Personally, even with the mid-end computer graphics, I'd pick the X360 over it.
Also consider that you don't have to deal with extremely low FPS like you do in a PC.