The only thing I can see speaking in favor of a Mac at the moment, is that Apple offers a system with retina display - but a quite a premium.
Not relevant as Photoshop CS6 can not utilize that increased resolution currently.
You can do a Forum search to find out more on this issue.
As Mr. Pfaffenbichler said - PS is not high-DPI ready, so "retina" doesn't matter just as it's not the best idea to run the programs on comparable "high density" PC monitors. Other than that you will probably not note any differences in Windows vs. OSX unless you get very deep into some very specific areas like the 3D stuff that may be hampered by the limited choice of graphics adapters on a Mac...
Thanks for the feedback, appreciate it!
If you choose a Windows OS, it should be a modern one. Photoshop CS6 won't do GPU accelerated things under XP.
Subjectively, based on what we're seeing here on the forums, it seems to me that slightly more Mac folks are having a little more trouble with Photoshop CS6 than PC users, but that could just be because right now Mountain Lion has introduced a few quirks, and Apple is still working things out.
That said, Windows 8 is about to release and that's going to cause a stir and an uproar the likes of which the PC industry hasn't seen yet. If you go with Windows, even a brand new system should probably be outfitted with Windows 7 for now, until the dust settles. Everyone is going to hate it at first, then the figuring out of how to actually make it work begins. I've already written a book on the subject, and it's already selling.