I just received GH3 for Xmas and initially I thought it was a "kids game," but by far it's the most addicting game I've played since, uhmm... I can't remember. Looking at the minimum and recommended system requirements for both Mac and PC, they seem to be fairly similar although as always, PC video drivers are better. Initially I was a bit skeptical of this game running on my SR MBP(2.4GHz) as I've had a few disappointing endeavors with Aspyr ported games, but GH3's performance has surprised me; Although one can never overlook it's rather steep system requirements.
Although I haven't played GH3 in Windows, I would go out on a limb and say that gameplay is probably smoother, although it's not too shabby in Mac OSX.