List the pros and cons then decide.
Pros:
PC: May run better, modding, mouse and keyboard (if you like that), no RRoD, cheaper, it was originally made for PC (franchise)
Xbox: Joystick (if you like that), ability to play it laying down, sick, just generally more comfortable. No system requirements problems, bigger screen, better sound (sometimes)
Cons:
The opposite of what's listed above for each thing.
Why do people think you can't use an Xbox controller with the PC version? I have my PC connected to a big HDTV, killer surround system, and I can use a 360 controller if I want to. I'm not weighing in for either side, but seriously, why do people think you're forced to use they keyboard?
Anyway, if I can get a game for the PC that's what I do, and if I can't then I'll get it for the 360. On the PC the games are cheaper (I just got Left 4 Dead 2 and Blur brand new for $20 for both, for example), can potentially have much better graphics and more going on in the game due to more processing power, can use the same TV, surround system, and controller the 360 does, etc.
The main reason I can see not to go with the PC is simplicity. There's no doubt that gaming is simpler and less of a hassle on consoles, and that's a 100% legit reason to prefer them. Most of my friends that don't game on PCs just don't want to deal with them, and that's fine too.