PC definately. Games are cheaper, can be optimised, can be played while working
The downside is the price of the PC, with a decent rig for modern games at around $1500 while similar graphics from the 360 at $550.
Although when you weigh the price of the console and the games together, the PC kills the 360, because you can pretty much get any game using torrents for FREE... But that is illegal and should not be encouraged