The best computers I ever used were dual-boot Macs. Best of both worlds. Hella expensive though.
Personally I prefer Windows, because I grew up with it, and the OS makes sense to me. I prefer the way the files are organized, among other things.
It really, really depends on what you plan to use it for. Do research, talk to people at MicroCenter, and work through your budget. Chances are that whichever you choose will end up working well for you.