For what it's worth I work in both environments and have never had a problem. I only bought a MacBook for live reocrding because I couldn't build my own laptop. I have built my own PC running Windows 7 and have no problems with that either.
I guess I would say if you can build it all yourself, go for it, if not, I would get a Mac. I am neither for or against either one. Just know what you need and make it happen one way or the other.
Fostex 812 Mixer -> MOTU 828 mk3 -> MacBook: C2D, 2.4Ghz, 4G RAM, OSX 10.6 || i7 920, 2.66Ghz, 6G RAM, Win 7 Pro -> Reaper v3.6