Its down to personal taste. I think Macs are great, but I prefer PC's.
Windows 7 with CS4 is superb I find. Havent encountered any problems. Its fast and does what I need.
The problem is, people rate a standard PC against a Mac. You can't do that. A standard PC from your computer market is NOT a decent machine to run specific requirements on. If a PC has the same spec as a Mac, both will do the designed job perfectly.
People buy a Mac at a high price built with high specs. People buy a PC at the CHEAP, they want a cheap machine to get online or whatever, this doesn't work if you then expect that machine to do a tough task of using Photoshop.
Photoshop will run crap on any machien with low specs. Give a machine decent specs it will run Photoshop well no matter what OS it has.
Yes I have used Photoshop on a Mac here at work, before the Mac fans say I'm biased or haven't tried it
BOTH are good, I wouldn't rate one above the other.