Mac, by far.
When it comes to recording and editing music, I've tried many with both Mac and PC platforms. And I've come to the conclusion that Mac beats the PC hands down when it comes to professional level recording and editing. I believe the Macs stable operating system has a lot to do with it.
I use my Mac with Pro Tools, which is used professionally throughout many audio industries for recording and editing music and more. There are so many items available with and for Pro Tools like virtual instruments, Midi editing and control, notation, and many other plug-ins from third-party publishers.
The PC has a very powerful studio system that I used to have lots of fun with called FL Studio. FL Studio also offers the ability to work with some third-party plug-ins. Their more popular entry level software is affectionately called "Fruity Loops".
If your into music editing as I am you'll enjoy anyone of these wares.