I always worked with both and quite frankly, both are tools to make the job done; being more a matter of user preferences and/or business model.
For example, it was common to see both in a collaborative environment: designers, retouchers, editors...on Mac while colorists, fx artists... on PC.
PCs were crap compared to Macs when it comes to independent professions, small facilities, because they needed maintenance and computer knowledge not every business could afford.
But nowadays one or the other really does not matter unless you have a specific reason: a proprietary software or media format available such as FCP or Prores, or the use of shortcuts etc...
Macs have generally a better built quality. I dropped one day a notebook that fell just on the edge corner: nothing! Not a scratch. If it had happened with a PC...well...
Then the crapfactor, which is psychologic. All the collective imagery that surrounds PCs was less sophisticated. Cool factor zero. PCs is popular and common, Mac is the desired object that could be exhibited in an art gallery. And that, from a marketing perspective, changes it all.
But IMO, the area where Mac has a great advantage today is not the computer world against PCs but the iPhone.