I use macosx and winxp.
macosx on my desktop dual 1.25ghz g4's, 1.25GBDDR, 80GB7200rpm
winxp on my asus m6n 1.7ghz P-M 1GBDDR, 60GB7200rpm
On a hardware note:
something you might want to consider about apple is that I've read that they are probably going to do one or two more powerbook g4 revisions (upgrades) before they bring out new powerbooks with somesort of g5 (this is of course speculative but nonetheless what's been going around the grapevine). So if you wait until Oct/Nov to buy one, they might push the g5's soon after you bought your g4.
If you buy a pc around the same time, oct/nov, you probably won't get hit with that kind of "being left behind the herd" as I've heard that Dothan will be around for a longer time (just being stepped up Ghz wise) and not replaced by an entirely new processor, (like the g4's will be with the g5's).
On a software note:
Choosing a platform may be influenced by what you're going to be doing with it.
Both OS's have their plus's and minus's and I suggest that if you've never used a macosx computer go to an apple store and use one for an afternoon or more. Same with a windows pc (but it sounds like you've used windows before), try it out before you buy it. You may be pleasantly surprised or radically disturbed by what you find just by doing day to day operations.
Another software note:
It sucks having to buy two versions of software. That alone could be a very influential factor.
I have a question for you, sbc, what kind of work will you be doing with your new laptop?
Here's what I do, and why I chose an ASUS PC laptop (over the PB15"):
Things I do that both ASUS (xp) and PB15 can do well:
Photoshop, Illustrator, Indesign, Acrobat, Dreamweaver, Text editing, web surfing, printing, word, excel, powerpoint, flash, mp3's, dvd's, premiere
Things that ASUS (xp) can do that PB15 cannot do at all:
Rhino3D, Unigraphics, Alias Studio (not the same as Maya)
The last three are a few of the main reasons I bought a PC portable over a Mac portable. So, your decision is at least partially dependant on what you're going to be doing with the system. I suggest looking at what software you use regularly.
-Hope this helps,