Here is my imput.
I am a pround owner of a 15" powerbook I purchased about a month ago. One of my best buds puchased a 12" powerbook a week after I got mine. I do LOVE the screen on my 15", it is wide, chrisp and amazing. Not too mention at night with the keyboard to lit up, it is just a most beautiful machine. The deal with the battery is I end up with about 3.5 hours maybe a little more....which can really suck at times. Portability has been an issue, in that due to the wideness of my laptop it can be cumbersum and a little heavy in my bag. I did purchase a crumpler sleeve with it which adds ever so slightly to the bulk.
I have to admit I am jelous of my bud in that his computer rocks when it comes to portability, and the size makes it ideal for carrying it around all day. I sense he is jelous of me cause of my screen and a few other high performance features I have. As far as battery life he beats me out by maybe thirty minutes tops, so really that is not that big of a factor.
So, in retrospect, if my current laptop was not going to be my only computer I would have gone with the 12"ibook, if nothing else for the portability and the kick ass battery life which is right at 5 hours. Of course owning a powerbook is an experience all onto itself. My next move will probably be to buy an ibook to carry around on campus or consider upgrading my current pc laptop to a 12" ibm think pad...the gist is either way you go, you will be a winner! Though if portability is key, def closley consider the 12" eitheir powerbook or ibook. Good Luck!