I agree DJ Sub, macs are most expensive than PCs...and I'm not really happy with that...but they make some efforts to make them more cheaper now...about audio softs, look at the main ones, they're all avalaible for macs (logic, cubase,...), and macs are also great for some experimental sound design (Metasynth, Cecilia, Max,...).
About OS, I used 8.1 (I got a powermac 7600/132...an old one)...and I can't say it's always stable, but I've good hopes regarding Mac OSX, a more opened OS and you'll be able to use linux base apps, a really good stability, and the management of multiprocs...
About hardwares and repair prices...I'm using my 7600 since more than 3 years, and I never had a problem with it, all the components are assembled and optimised by Apple.
I know that PCs are now much, much better than before, and less expensive. I think I'll change my computer soon and I don't know if I'll stay on mac or not if I take a desktop, but if I go for a laptop, Mac are really better than PCs regarding the specs and the price.
I'm a Mac lover, not a Mac fanatic. I won't say that PCs suck, both platforms have their advantages and disadvantages, maybe Macs are just more easy for newbies.