I just re-did a comparison check.
With Vista home premium 32 bits I get a higher framerate with everything maxed and on (Glow, multisampling etc.)
The lowest my fps went this morning was 57, in Orgrimmar with a lots of people on screen.
I rebooted in OSX, and checked at the same spot with about the same amount on people on screen, I had 54 fps.
I play the OSX version anyway because even though it doesn't perform as good as the Vista's version, 3-4 fps is not something you'll notice. And in both version the framerate is rock solid, I never dip below these values, there is no hitching. No matter what I do, where I go or what is going on on my screen, the framerate stays always above 50 fps (and that's when things get really crazy).
I use Vista mostly for games that aren't available on OSX or that performs really poorly on OSX. I couldn't imagine myself playing WoW on windows, I like those Mac specific features and since I play windowed most of the time, I don't have to put up with Vista.