My preference: Mac
I've been using Apple computers for almost 2 decades in both personal and professional use and I've no regrets.
Mac OSX is a rock-solid UNIX-based operating system and their hardware, especially the Mac Pro, is some of the most powerful you can buy.
Yes, you can probably buy/build a decent Windows-based computer for music and probably do just fine but my experience has not been that, overall. I've experienced more downtime and time spent trying to correct problems on Windows-based a/v workstations than I ever have on the Mac.
I've been through the drama of having several custom Windows machines built to do nothing but a/v work and wasted more time than I spent working.
Bottom line: the Windows operating system is full of flaws and security issues and the associated potential hardware inconsistencies sure doesn't help the matter at all.
If you stick to excellent-quality hardware and keep your Windows install clean and stripped-down then maybe you'll have something to work with...or just buy a Mac and get to work.
