Windows: Crappy, slow, virus infested, but almost everything can run on it and it is also the most popular and most favored
Mac: Fast, smooth, not nearly as many viruses, but isn't compatible with very much (that is NOT Apple's fault - it's the designer of the program's responsibly to do it) and is rather rare and unliked
I personally would take a Mac that runs Windows as well. That is a huge advantage as then you can have a Mac AND Windows on one fast computer. Windows can't do that >:D and if you're not gaming then yo don't even need Windows.