Both actually. But I'm biased. I love OS X and I'm not too keen on Windows. As Forty2J mentions, performance under OS X is definitely acceptable, and in fact can be very enjoyable if nitpicking performance simply isn't your priority. I personally really wish I didn't have to have Windows, but as mentioned again, it's because of the game selection. It's the only reason why I am tethered.
I seriously don't think you'll mind sticking to OS X for gaming in terms of performance. There's no doubt on these forums that they run better on Windows, but they still run well on OS X with the 680MX. What matters most I suppose now, is just the question of if you'd be ok with the smaller selection of games on OS X and the lag time between a game being released on Mac after it's been released on Windows. Although Aspyr has gone on the record saying they're getting much faster at churning out titles. If you also look at Feral, they too seem to be accelerating development times. And both Blizzard and Valve release both Mac and Windows versions day and date on launch. Already got Heart of the Swarm pre-ordered in fact!
I own a lot of titles from the Mac App Store and it's really great to see the growth of game titles expanding faster than it was before. Same thing with Steam. And whether you love them or hate them, EA is also scheduled to release Origin out for Mac next month, so I'm really hoping to see some big EA franchises finally make their way to the Mac.
I can't wait for that day
Aspyr recently did a cool Q&A with 9to5Mac. (
http://9to5mac.com/2013/01/16/aspyr...ac-gaming-borderlands-2-giveaway/#more-257967)
Both production times and selection have been ramping up so the future at least looks bright.