Normally I would prefer getting the Mac verison of the game. And if the reviews of the Mac version are similar (or slightly less) I would definitely prefer to buy the OS X version, not only so that I wouldn't have to (re)boot into fugly Windows, but also to support those Mac game devlopers out there.
OTOH, you would be nuts buying a crappy port of a 2 year old PC game at a premium price. 😛
Reminds me.... FERAL, WTH IS COLIN McRAE 2005????