For most purposes, there isn't much difference except that the Mac will be more reliable, easier to set up the way you want, will need *much* less maintenance, and will have a user interface that gives you less eyestrain. This goes for all the *common* applications like word processors, spreadsheets, graphic software and software to do stuff over the Internet.

As someone already pointed out, games are an exception; Windows machines are better supplied with games. The other main exception is sound processing software. Some of this is *not* available on Macs, and if you're already committed to a particular package, you might end up stuck. I have the opposite issue - the main program I use is BarFly, the ABC processor for the Mac, and there is nothing comparable on Windows.