I hope this thread shows just how subjective it is.
There are some cases where one OS has a pretty strong advantage. If you need to run engineering software, choose Windows. If you are in academia or the arts, consider Mac. If you know that you experience more than your fair share of weird computer drama, get a Mac with Apple Care. My friends who just don't want to be bothered with computer innards feel that they are taken care of by our local Apple dealer, who really goes out of his way to pamper his customers. Plus he attends my gigs.
Other considerations: How much do you want to spend? How about size, battery life, etc. Is the "cool factor" important to you?
The OS wars are gradually becoming less and less important to me. Every OS is windowed. They all connect to the Webz. The mainstream software apps (browser, word processor, spreadsheet) are remarkably similar if not identical. I used to think that Windows was more developer friendly, but I have gradually moved away from traditional programming in favor of higher level problem solving tools that are platform independent. So, the cooler I get, the less important my OS is.
Now, I can see running Mac or Windows, but both? It would seem like a bitter irony to pay a premium for hardware in return for the privilege of buying and maintaining not one but two bloated proprietary OS's. I run Linux for Internet related stuff for security reasons, and Windows for the engineering stuff.
