At this point, the 12" PowerBook and 12" iBooks are pretty much equivalent. I tend to be somewhat leery about the iBook line because of all the issues Apple had with the iBook G3s, but the iBook G4s seem to be a solid product. I anticipate a new set of Powerbooks coming out sometime quite soon, because the iBooks have been improved so much (they got the sudden motion sensor, the scrolling trackpad, etc.). The 15" and the 17" are different animals; much better graphics chips, much higher resolution. Given the choice, I would take those over a 14" iBook any day. The 14" iBook has the same resolution as the 12" iBooks-1024x768.
The other advantage the PBs have over the iBs is the fit and finish on the PowerBooks is really quite nice. I'm using a PBG4 12" for work right now, and I am pleasantly surprised by the feel of the machine.
In any case, if you can wait, I'd wait until after Apple Expo (September 20) to buy. Of course, given Apple these days, they could bring out the new models on Tuesday, or wait another 3 months. Who knows...
