Computers are tools, kind of like screwdrivers. What's the difference between a flat screwdriver, a hex head, and a phillips head screwdriver? Not much, unless you are dealing with a specific type of screw.
I think the real question is what are you doing with your laptop (what kind of screw)?
Email
Web
Word Processing
Digital Photos
Ok, if just that, then Mac or Windows--totally up to you. Whatever you find easier, cheaper, etc.
CITRIX connections to the office system
Propreitary software for work
Games
If some of those features, you need to look at what operating system is supported by those programs (and it will probably be XP). There are Mac substitutes for most major programs, but there are some small proprietary software programs that are XP only and if that is related to your job, well then the answer is pretty clear.
But something you might not be aware of is the fact that the Mac can run the Mac OSX operating system and the Windows XP or Vista systems AT THE SAME TIME. Using Parallels, you can switch back and forth between operating systems as needed. One of my Macs is hooked up with OSX, XP, and Ubuntu Linux, and I can switch between all as needed. You can't do that on a Dell.
I think you'll find a lot of people agree with Dar. They went through some growing pains switching to OSX, but after awhile things just were far more intuitive on the Mac then they were on XP or Vista. Little things just started to make sense, and doing other things like connecting a digital camera or installing new software were just simpler, easier, and somewhat more elegant.
"It just works" is often a phrase associated with Mac users. I can tell you it doesn't always 'just work' (but then again I do some pretty intense stuff on my machines) but if you are doing some basic computing needs, you might want to see what the Mac can offer.
There is also the issue of security. While no system is safe, the majority of virus and spyware is written for the XP world. There are also some things done on OSX that help minimize damage should a virus come out (such as root disabled by default). Do a search for OSX XP switch and you'll see some interesting stories.
(I have 10 computers at home: seven Macs (three with XP on Parallels), 1 XP computer, 1 Centos/Linux server, and 1 tiny embedded linux box).
p.s. if you haven't used a Mac in 10 years, then you haven't used OSX. It's a world of difference from the OS8/OS9 you probably used before. Worth seeing at the Apple store if you get a chance.
p.p.s. My other word is 'why a laptop?' If you travel a great deal and need to be online, then I can understand it in that situation, but by and large laptops are lower powered, more expensive, and far more likely to break than their desktop cousins. Depends on your needs.