I'd personally get a Windows one. Almost all programs that'll you'll use will run on Windows. Windows XP is very very stable.
I have not had ONE bluescreen since I installed it around 3-4 years ago, and I'm not joking.
As, Troy said, you could get a Mac and run Windows.
But, put that another way, you could buy a Windows laptop or PC, and run the Mac OS...
Personally, I'd rather be able to upgrade with cheaper parts, and also, be possible to run games using a good graphics card. Something Mac's aren't really proficent at doing.