You know, it took me a while to like Toshiba...but after much research...I just fell in love! I own a PowerBook G4 as well..I love them both....it's all about preference and about the type of things you will be using them for.
My father has about 4 kick around business Dells that he's got wired in his whole house not including desktops. I think they're Inspirions? Anyway, Dells are pretty good for overall value, quality, and support (that's their #1 thing). But I have a Toshiba product replacement plan only for the fact that I dropped some cash on it and I have the same for the PowerBook but got a sweet deal on it as well.
I look at IBM's as more of the business side of laptops to tell you the truth. But as a designer, it's about performance/power. It would be nice if they'd hurry up and pack in some power in those Tablet PC's!! Now those are great (Tried Alias Sketchbook Pro..and it's wonderful on it!) Til then, I'll stick to the Intuos tablets.
As far as weight goes, that's personal preference as well. Most people now days have things on rollers. I stuff everything in a skateboard backpack! Those things hold everything. But I have seperate sleevecases for everything as well, if I don't need everything.
I just don't recommend HP/Compaq for what you will be using the laptop for...I haven't heard great stories of longevity as far as the hardware goes.
Check Pricegrabber.com for pricing....if you're a student by any chance...you can get some great discounts but if not, discounts are still there...or incentives...extra ram..etc..
Get something that you can live with for the next 2-3 years though...those new Dell laptops...comes with the upgradeable graphics cards...which I wish would've came out when I decided to get my Toshiba!! But oh well....so definitely look into all your options and good luck!