I've had both and currently love my 12" Powerbook. However, for your uses, the iBook seems like a better value. If you're going to be carrying it around a lot, you'll appreciate its greater ruggedness and slightly better Airport range and battery life. If you're going to use it with an external display, however, go for the Powerbook if the display accepts a DVI signal -- I found the DVI connection produces a sharper image, and you can't get it with an iBook. And the PB has a better keyboard and bigger HD (unless you BTO a larger one on your iBook).
For your uses, the PB speed advantage is minimal compared the performance increase you'll get if you get adequate RAM (like 768 or more).
iBook updates are rumored soon, so you may want to hold off a bit if possible.