Cost:
PC's are much more cost effective [duh]. Components can be upgraded fairly cheap and you can find parts for PC's, seemingly, on every street corner. Where as, with a Mac, you can't upgrade anything outside of Ram, Peripherals [mice, keyboards, etc.], and Drives. That pretty much makes Mac's a disposable machine -- if you want a faster processor you have to buy a higher grade Mac.
Performance:
The bit about Mac's being faster is a myth. That is until they switched to Intel Chips. I have both a g4 Powerbook and a 1.83ghz Intel MacBook Pro -- there's a notable difference in the MBP in comparison to the Powerbook [even when running apps in rosetta]. Subjectively speaking, my MBP FEELS faster than my 2ghz PC. Let me add that i have 1.5 gigs of memory on my MBP.
Reliability:
PC's are a little harder to deal with. There are literally thousands of vendors for PC's and PC components. And naturally, to release their products before the next guy they relase alot of products before they're completely finished, leaving the user to uncover the problems. So, in theory, you've got unfinished components packaged in PC's, meaning it's hard to tell what component is f'd up. With a Mac, the most important components are from Apple, working with Apple products, so it's less likely you'll come across problems. And if you do, Mac's probably heard about it, being that everyone has the same components and therefore a lot of the same problems.
As mentioned before there are thousands of viruses and worms for PC's, very little for Mac's -- i don't have any virus protection for either of my Mac's and probably a couple of viruses [if not, then spyware] on my PC WITH virus protection.
Mac's have this category won hand's down, imo.
Music:
This ain't the 90's anymore, where any studio that was doing something major had to have a Mac. You'll find both PC's and Mac's in studios now-a-days with regularity. You'll probably see that they have both to cater to all types of clients, so I can't exactly make a case that one is better than the other anymore. But let me just say that in dealing with a Mac and dealing with a PC, it's almost ALWAYS easier to "plug and play" with a Mac using the Audio Midi Setup App enabling you to define more parameters, where as with PC you have to use the [limited] Control Panel Sounds directory.
Conclusion:
I guess if you put a gun to my head i'd say Mac if you can afford it but it's not as if PC's are a consolation anymore. If you have a PC and like the way it works then don't fix it, spend that hard earned cash on some other gear. One thing to note, which is why I haven't totally switched from 12" Powerbook to my MBP for audio, is that not all software companies have switched from PowerPC to Universal Binary.
Sorry for the length, this shiet can't be said in a couple phrases, in my opinion.
Krack