I run windows vista and 7 on my mac pro and XP on my MBP but for games that run on both Mac and Windows, mac osx will achieve a FPS 2-3x higher than windows. I would hope that the more recent games would close that gap. I do still prefer Mac OSX and find my self using windows less and less ever year.
It will be interesting to see what your son does by the end of the year with the Mac App store coming very soon. Microsoft better have something up their sleeves otherwise I think Windows will be left behind.
BTW, check out steam for games to run on both platforms