I recently helped an ex-girlfriend make exactly this choice...
Honestly, the 12" powerbook vs. 12" ibook I feel like is a tough choice given the price differential, but the 14" ibook is basically the same price as a 12" powerbook once you upgrade the ram, and in my opinion is a totally inferior machine. The 14" ibook is heavy (more than 6 pounds, I think), so it's not great for lugging around. The monitor is bigger, but has the same resolution as the 12" powerbook, so you see just as much, even though it's smaller on the powerbook. It's slower, you have to do the spanning hack to plug an external monitor into it and do anything but mirror, and it has no DVI out, so the image quality on an external LCD is inferior. It also has less video ram, so if you do plug in a big monitor to it, expose gets all jerky (like with the mac mini). I just don't see any advantage, aside from a marginally bigger built in monitor.
If you're comfortable at the price point of the 14" ibook, then I'd really suggest getting the 12" powerbook instead. Use it for everything, and then if you really find the display too small for home use, wait until the floor drops out of the prices on those dell 20" widescreen LCD's ($325 a while ago vs. $800 for the apple version), and plug the dvi out of the powerbook into it. Best of all worlds -- totally portable, great battery life, and a monster great looking display at home.
You asked for opinions...
I have a feeling I'm about to get flamed by everyone with a 14" iBook... Let me be clear that I like the machines a lot, I just think the 12" powerbook is superior at the same price point.