I'm a PC gamer, because I don't own any consoles. I'd certainly like to own an XBox 360, because with a growing sense of dread I am becoming aware of the fact that more and more games are coming out for next-gen consoles and not for PC. Although I really prefer mouse/keyboard over a controller, but that's only because I've used that all my life.
I also would like a console because if you get an XBox 360, it has set "specifications" so any game made for it will meet those specs. In other words, on a console, you won't have lag because your processor is too slow for that game, or graphic problems because your video card isn't good enough. PC users always have to upgrade, as someone else already mentioned.
Nevertheless, PC gets all the mods/downloadables.