It would have to depend on the game and your computer. I would research other users experience with the Mac game on machines similar or identical to yours. Some games from Aspyr are poor ports in my opinion and as a result crash and overall provide dismal performance. Bear in mind that some of the games for Macintosh platform are priced ridiculously higher than their PC counterparts. In the end it would have to boil down as to whether the Mac port was playable on your computer and you how willing you are to reboot into Windows just to play a game.
I'll wait for reviews, to see which has better quality and is more stable.
(But for those using Cider, the games are translated on the fly(right?) and that will probably lower the performance.
I've another question.
Is the SOURCE CODE needed to PORT a game?
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.
My personal opinion, as much of a Mac zealot as I am, is very similar to the first poster--look at other experiences. What tends to happen (though this fluctuates wildly) is that many cutting edge games (oh how I despise thee, DirectX) will run better in Boot Camp, while older games run just fine in our native OS of choice.