It really depends. I went with the 15 inch due to wanting the higher screen resolution, since the Powerbook has become my main computer. But the 12 inch might be right for you. Best thing as Bitt said would be to play with them at the Apple store. Screen size is a personal preference, one that you should make standing in front of the real machines.

As far as preformance on the 12 inch, the first version did have some issues. They put too little cache into the CPU, and ran the bus slower. This ended up causing the Powerbook to be slower in some things then the iBook. Newer revisions fixed this by using the same CPUs as the bigger Powerbooks. The first revision also put out a lot of heat, something that also has been corrected in the newest revisions.