12"
I use the 12 inch. If you're a user of the 12" you'd never concede that the 15" is "just as portable." While the 15" will fit on a desk without problem, I can use the 12" literally everywhere. This thing is the size of a school notebook. The only time I find the screen small, as I remarked in another post, is after using my boss's 23" Cinema display. Yes: I am doing graphics and web work on a 12" screen, and I have absolutely no problem. You can probably use the 15" book anywhere, but with some effort, no doubt. I had a 14" or 15" PC notebook a year back, and I couldn't do nearly as much with that as I do with my 12" PBook. Working on the train, watching DVDs in the air or car, using it on my lap anywhere in the house - I can't see anything being better suited to portability, without sacrificing much else, than the 12" model.
Don't get me wrong; the 15" PBook is very cool. A coworker of mine uses a 1 GHz version, and I think it's one cool looking machine. But it's nowhere near as portable as the 12". If you're looking to use your 15" at a school desk or at a desk in your dorm, you have found your match. But if you're looking for a computer that you can use anywhere, 12" is the way to go.
Mike LaRiviere