The 12" is definitely more portable. However, the 15" is definitely not "cumbersome." I have a 15" TiBook, and I have no problem carrying it around, though I guess 12" would be easier. Note that some people have complained about the quality of the screen on the 12", comparing it to the iBook screen. The 15" screen definitely seems sharper overall. If the screen doesn't bother you, you might even consider an iBook. Apple just released new ones today that look mighty tempting, and at half the price of the AlBooks.
The Powerbook G4's are marginally more upgradeable than the iBooks. No laptop is really all that upgradeable. In terms of ports, unless you do lots of high-end video/audio work, you ought to seriously consider an iBook. The money you save by not having all the bells and whistles can be used for more useful stuff, like more RAM or Microsoft Office or Airport hubs and whatnot.
Whichever you decide to get, do get a sleeve for it. You'll want the added protection because they scratch up easily. You can get one for around $25, made of neoprene. Also, consider getting something to protect the screen when closed. The keyboard will mash the screen and leave marks otherwise.
PM me if want to know more. I apologize for the choppy post but I'm trying to outline for a paper.