Very hard question, both are very good System, I like creating your own system which is why i like PC’s. If you know what you are doing you can get bits and pieces (a wide range to pick from as well) and produce a awesome system for cheap, while macs’ all ready system are good but pose problems when trying to upgrade.
OSX on the mac I believe has a slight edge on XP on performance and its definitely much nicer looks / animation / feel (just love that interface). But windows and PC has always been superior to things like 3d, gaming, Compositing and working as Servers.
I use XP and it has been very steady for me , and has not been a problem since i installed it, Ive always been a PC person because im really use to the logical and quick way around your computer while i still love the look of OSX, that dosent bother me because you can install things like StyleXP and found on the web some excellent skinners who have developed themes to make your pc look sorta like the OSX interface.
Macs’ are known as the design tool. I do agree in some aspects if you only use the Studio suit, they can handle these application very well, but i found its lacking support for other application which i would use. eg… 3d…composting apps, 3rd party apps.
As powerful as macs can be they still run Flash slower than a pc can run it. They are not too bad while working , but when testing, you will never get the real runtime that you would on a PC, i believe is something to do with the allocated memory… but im not sure.
So in conclusion… both are good. one does some things the other cannot and visa versa, it up to how you work and what you need in a computer , that will finalise your decision.