Get the 15" PB. You get more speed, a larger (wide)screen, better resolution, and the best looking laptop in the world...
I am getting one myself, and I did not even consider the iBook. However, I do not really care for ruggedness and portability, as I am going to use it as a desktop replacement.
You could consider an iBook for the short term and saving a bit more to purchase a G5 PB when those arrive (since you are already planning to replace your iBook in 1 1/2 years). But 1 1/2 years is quite a long time when the iBook does not turn up what you expect it to be.
It seems to me you need the additional performance of the Powerbook. Go for it.