Either is just a tool, either will only be as good as the person using it.
Macs have a better operating system (unix based etc etc) bit PC’s are a lot easier and cheaper to repair and buy in the first place.
Design studios use macs mainly for the following
1) Unis have historically taught on macs as the software was originally aimed at macs GUI rather than PC’s original command line interface. This has simply carried through.
2) Kudos, macs are perceived as trendy and different, if everyone had/used a mac the uber trendy would have to find something else.
It really depends what our going to do on it…
For games macs are next to useless, for viruses (so far) they are far more secure, at least until hackers get round to bothering to write a mac virus.
For office admin tasks PC’s are probably more accesible, accounts packages, contact management systems, office suites (open office is free and very very powerful if no one has already mentioned it) etc.
For design, very little in it, maccs probably just have it for traditional print based tasks and for mid range video but for web it is about equal. We run Adobe CS3 (webdesign premium)and experience no problems with any of the elements.
As we are historically PC based (not from a traditional design background) the cost of changing all the machines and licenses makes no sense… would I like a mac yes, it has that indefinable something, but it is a like not a need.
As I said for most of us there really is little either machine will not do, hence both sides aggressive marketing campaigns, both probably know this.