zipper,
I considered the 17in too and went with the 15in. I did this because the 15inch is much more portable, the 15inch has a great screen resolution (the last Powerbook models like mine. Also the 17in was quite a bit more expensive and the difference in price didn't justify the bigger screen, and the additional bulk/size of the 17in.
It was a better investment in my opinion for me to buy a 20in Widescreen LCD which I could hook the Powerbook (and any other computer in the future) up to. That is what I did, and I'm glad I made the choice. I now have a black MacBook with a 13.3inch screen and I love it. When I come home I just plug in the DVI connection and I have 20 inches of screen real estate. The widescreen LCD will be good for years to come with many different computers, whereas the 17in Powerbook's screen, while quite impressive is unfortunately attached to the Powerbook. When you're done with the PB, there goes that nice screen, and you'll have to purchase new 17in MacBook Pro at a premium.
I'm one of those who thinks laptop = portability, desktop = more power (processor wise, screen wise, etc). Though that gap is closing speed wise, the 17in Powerbooks and MBPs just seem like portable desktops to me. All the friends I have with 17in PBs or those huge 17in Toshiba Satellites let them sit on their desks; moving them only when when go to and from college. Alot of them won't even bother bringing it home for Christmas break.
Thats just the way I see it, if anyone disagrees please chime in. My friends might just be really lazy, and I might have just really wanted that 20in widescreen. =P In any case