Well the option for both wasn't considered, so I can't vote. I definitely use PC for a majority of my games, but I've got a Mac for work and after they released Steam for it (and HoN) I've been known to open it up to game on the go.
Well originally I gamed on my PC, but it started smoking one day when I turned it on and never got it fixed, so i've just been gaming on my 27" iMac and use to macbook pro when I'm just chillin to surf the forums.