I have a 12 inch iBook, which is why I recommended the 12 inch Powerbook...
Don't get me wrong - I dearly love the lil' thing, but the mechanical design isn't as solid as the Powerbook. In particular, on the iBook, Apple uses a single, narrow, plastic, center-mounted hinge to join the top and bottom together - not a very strong arrangement. Nobody uses just one small hinge to hang a fence gate or barn door, and for good reason!
On the Powerbooks, it's a wide, metal hinge - not quite as solid as using two separate hinges at the extreme corners of the case, but still a major improvement.
Also, the G3 iBooks had a persistant problem with the logic board self-destructing every 6 months or so. Apple has a repair program in place that fixes them for free (even if they're no longer in warranty), but it's still a pain to be without your computer for up to 2 weeks. Reportedly, the G4 version uses an entirely different logic board, and seems to have been trouble-free so far. The Powerbooks, meanwhile, have had a spotless reliability record all along.
Powerbook's just a little beefier design, IMO.