I use Windows Vista, XP, and Mac OSX on a regular basis and I have to say that PCs are better computers.
The main arguments that Macs are better are that:
1) They rarely get viruses
2) They are better for artistic and musical purposes
Both those reasons are poor arguments because:
1) Virus avoidance is easy with McAfee Site Advisor and common sense. I have not gotten a virus in over 4 years. (Sandboxie is another great program but its more for the techies than casual computer users)
2) PCs have the same programs that Macs do (the Adobe Suite, Autodesk, etc) or they have alternative and superior programs (Fruity Loops has much more content and is updated more often than Garage Band)
Plus PCs:
3) Have better hardware so running ANY program is much smoother and faster
4) Have much more compatibility with peripherals and software
5) Have a greater quality and quantity of games
6) Are cheaper
The better hardware is a huge deal for me. Macs, with their inferior hardware, are more prone to freeze than PCs and cause a great deal of frustration an inefficiency.
I am a graphic designer, soon to be game developer, and I prefer PCs to Macs.