1. "This is kinda a complicated question" In response to In response to 0
Generally I'd think a desktop would be best because you can upgrade the hell out of it more easily. BUT, you're talking about getting a Mac so I don't know if that's quite the case. You should consider how much extern stuff you plan on adding to it. How portable is a laptop if you have a few midi controllers, and external drive, and an external audio interface? It's up to you though.
Also, I would suggest that you stay away from G5/G4 computers. There is going to be pretty much NO new software for those systems due to the Intel switch. If you get an Apple desktop, wait until they drop some new shit.