I prefer Linux over XP.
Or more actually Debian over XP. It's just a more capable and flexible peice of software, you can do things with Debian that is impossible to do with XP, but XP does most of what most people want.
Learning Linux can be very rewarding, and since Windows XP and Linux share most of the same basic concepts you can learn a lot about both OSes. (ever try to learn OS/400 and you'll see how similar they are. WinXP used a lot of ideas from Unix.)
If your actually interested in OS design, Linux is were it's at. It's easy (relatively), accessable, and completely open. If you just what something to use, then use whatever. Some people like Linux more, some people like Windows more.