I could be biased and say a Powerbook, but there are a few things that you have to consider... mainly what are you using it for?
for any sort of photography or music editing purposes, I prefer OSX (thats a mac, incase you are wondering) because I find that they are much more stable. I have never had a virus on any mac, never had anything crash, and everything is idiot proof with OSX... and it actually works. My BF used to work for HP, as an example, and I was told that windows XP has a lot of printer-related bugs, if you want a solid evidence based example.
however, if you are more into games.. I will be honest, a windows based machine is the way to go for sure. There just arent as many, or as high profile games available on mac yet.
This all being said, I love my 12 inch powerbook... I took it with me to the last wedding I shot, and it is so small that it fits into a normal messenger bag without having to use an ugly laptop bag (although lately I HAVE taken to using a hardshell bombproof case for it...but I keep some of my camera gear with it now too... so it'd better be sturdy!) I have it hooked up to a second display at home, which is quite a nice setup.
macs are more pricier if you are simply looking at specs ... ie ram and processor speeds, but more efficient at running programs.
The other day I had safari (a web browser on macs) , photoshop, msn messenger and microsoft office running, and I also had 80 pictures open in photoshop...Ive gone up to 200 images in photoshop, and i love love love how well this baby runs. my powerbook is the generation before the current one, which was released in June. it has a 1.33 processor in it, and 700+ mb of ram. cant recall the exact number.
as far as laptops go, you sort of have to go in, see them in person and play around...if you get talked into a pc or a mac by one person or the other, you may regret it in the end...they make stylish choices for both, but in my opinion you cant really beat the simple beauty of a powerbook...100% brushed metal... no plastic...and a glowing white apple icon on the cover. the ibooks are also sexy, with the modern white treatment.
good luck, let us know what you decide
ps, can you tell I'm a techno geek?
