I'm sort of torn between the 12" and 15" powerbook but I think the 12" will win
I have considered getting an equvalent ibook and saving some cash but the biggest issue I have with them is they can only (currently ) take a max of 640mb or RAM compated to the iBooks 1.25Gb. They also don't come with a superdrive, which is a biggie as I would like to be able to burn to DVD-R.
The biggest issue with the 12" powerbook is the lack of a pcmcia/card bus slot, which means no external cards, although it does have a modem, 10/100mb nic as standard and the option for an airport extreme card. The 15" has pcmcia slot and a gigabit nic,
As nice as the 15" would be I've never really been a fan of large screen laptops and feel that although the 12" would be my main machine I can always plug it into an external monitor/keyboard and mouse.