Spriggan has some good advice.
Realistically, for most people, there isn't too much of a difference. I spend my day working on every kind of computer OS you've heard of, and often ones you haven't. After a while, none are really absolutely better than any others.
It comes down to what you are doing with it. If you are just going to surf the web, check email, play a few simple games, store pictures, etc, than it shouldn't really matter. Both systems have more than enough to do that. If you like using a Mac and don't see any problems with it, than stick with it. If you want to try something new and see what all the buzz about Vista is about, then give it a shot.
Even better, for many of those things that most people use a computer for, there are plenty of free software programs that are easy to adapt to and let you do what you need to do just fine. Many of those programs support both Windows and Mac OSes (as well as Linux). From that standpoint, it matters even less, because you can get the programs you want to use on both of them.
If you like iPods and are thinking of getting an iPhone or apple tv at some point, than it might be good to go with a Mac (even though both will probably work fine with Windows).
It all comes down to usage. If you like playing games, most people stick to Windows, though if you only play certain games (WoW comes to mind) than a Mac will work fine.
All that being said, there are a reason Mac laptops are so popular. Thus, if you are looking for something portable, and already familiar with Macs, than I would stick with those unless you have a really good reason to change. There are a reason you see so many Mac laptops around.
