You have to compare apples with apples
Up until PS CS2 the Mac version was optimized for PowerPC based CPU architectures. When you'd use this on the Intel based platforms like the newer iMac's the performance on a Mac was worse than on a PC due to the conversion of the programme via the "Rosetta-layer" which translates PowerPC like instruction sets to the Intel CPU.
As from PS CS3 Universal Binaries are used which negates this difference and so performing all the same. I am a Mac user, so I can't make a honest comparison between Mac or PC. I think that as long as you have enough RAM and CPU PS performs well on both platforms, but there are surely some aspects which perform better on a Mac than on a PC.
Overall the switch for me from PC to Mac was one I never regretted.. But that's another discussion almost as fiercely battled as Nikon vs. Canon users
As far as PS/editing goes, it's still the man, not the machine..