Depends on what you need it for. I've been comparing the two for a few months now, and I'm probably going to spend the extra money on the PowerBook. For me, the key differences are:
- Dual-display. I give PowerPoint presentations at work. I really want (close to need) the ability to use the presenter tools, where I can see my notes and the next slide while the audience only sees the current slide (via a projector attached to the external display port.) I know there are hacks to get this on an iBook, but I'd still prefer using a supported configuration.
- Audio input. When I'm not at work, I plan on attching it to the line-out on my mixer board (which connects a microphone and a few instruments.) Without this port, I would need to buy an external audio breakout box. I may end up getting one anyway in the future, but with a PowerBook, I don't have to get one as long as I don't want to record more than two channels at a time.
- More speed for the size case. The 12" PowerBook is 1.33GHz. To get that speed in an iBook, you have to get the 14" model.
- Larger standard hard drive. 60G vs. 30G. Everybody I've spoken to agrees that a hard drive upgrade in an iBook or 12" PB is not a recommended do-it-yourself project.
The iBook that comes closest in specs to the 12" PowerBook (same speed, same size hard drive) is the 14" model. This costs $300 less, and still doesn't have an audio-in port or dual-display support. Maybe you don't think the differences are worth that much, but that really depends on what you want to use the computer for.