The best answer I can give you is this:
Sit down and make a list of software you need to run. Once you have the list compiled, Look at any pieces of sw you would like that are platform specific. By that, I mean sw that will only run on a PC or Mac (Logic and DP on Mac, FL Studio on PC). That will determine which to go with. If you end up with all sw selections that will reside on either platform (for instance PTLE7.1, Cubase, and Reason), your best bet would be to give both OS'es a test drive. Give it some ample time too, find out how intuitive is the interface TO YOU (screw what everyone else says, you have to live with this decision, not them). How easy is it for you to navigate around? Look at the cost of peripherals if you need them, how easy is it to upgrade? These are the things you need to look at and once you do, you'll be able to make an informed decision.
Personally, I use a PC. I've owned PC's, Macs, SGI, and Sun machines in the past. PERSONALLY, my Macs saw the least amout of use because I simply didn't like MacOS (OS7-9.2.2). I was also a PC/Mac technician for a large (global) advertising/marketing firm (my branch did all of Daimler Chrysler's advertising and marketing, our New York office is responsible for all of the glitzy Pepsi commercials). When we got OSX in house, I had determined (FOR MYSELF) that the poor performance was not worth it. I spent weeks with OSX and ultimately got rid of it. It looks nice, but eyecandy does not make for a better OS, just a better looking one. Same for XP, when we got it in, my first complaint was it's poor performance and the eye candy, all wasted clock cycles. In the end, I rolled my laptop back to 2000 Pro.
The only reason I use XP now is because it's required by ProTools. If it weren't for that, I'd still be using 2000.
