Definitely windows, as it has much more compability for lots of programs. Despite the now and then occuring windows bugs ... Mac is very stable, but compability is less ...
Windows? Mac? Slackware my friends.. Slackware.. ^ Linux is by far the best, unless you like the pointless eye-candy that takes up "17GB" of your HD, when you CAN have that as well as the most customizable OS. "Once I changed to Mac I never looked back", well look to a linux distro. and you'll never look back [period].
I take back what I said about Apple. I just got to experience mac again today, and just remembered the google stuff they had since '01. Down goes microsoft!