Either one is just a tool and will do pretty much the same things. To me the big factors are:
Service and support are important;
for Macs is limited and Apple will have you by the throat
for PCs wide support (some bad, some good), Dell for instance has very good support, HP has very bad support, but these things change and next year it might be the other way around - still PC service is much more available and cheaper
Price;
PCs are much cheaper - but beware because some of the really cheap ones are junk. Get a good one from a reputable dealer such as Newegg, WalMart, etc. Talk to friends about their experiences as well as this forum to pick the exact model. The particular model and options are probably more important than the brand.
Ethics;
Apple has established a well-earned reputation for screwing their customers, dealers and anyone else they can, at every opportunity.
Of course some PC manufacturers like Gateway, Compaq(HP), e-Machines, etc., have also.
Every experienced computer user/service tech/designer/vendor/buyer/ can sing the praises of some and provide horror stories about the others