As the the PowerBook has a G4 processor, which is less powerful than a G5 which is slightly less powerful than an Intel Core Duo chip I would have thought you would go for the more powerful of the options.
I agree with this. I'd probably say it's about a fair trade. the iMac will definitely be faster, but the Powerbook has 2gbs of RAM and is plenty fast itself. If you need portability, you're making a good choice.