15" PB vs. 12" PB vs. 12" iBook: A personal experience
I've had a 15" Rev A Powerbook on loan for over a year and a half, and I recently had to buy a replacement machine because I had to return it. I tried out a 12" iBook G4 for a few months, and I finally ended up selling that and buying a 12" Powerbook instead.
My reasoning for this is that performance-wise, the 12" Powerbook and the 15" Powerbook is comparable, and the main difference is the screen real estate. I found that I primarily used my 15" Powerbook as a desktop replacement, hooked up to a Dell 2005FPW 20" LCD screen, and as such, for my purposes, a 12" machine would be most cost effective, since I have the portability I crave on the road, while having the ability to drive a large display while I'm back at my desk.
I am aware that there is a hack to allow the iBook to drive an external monitor, but it would only allocate 16 MB of VRAM to the external, making it very choppy when used with Expose. In addition, when connected to an external display, I prefer to use my machine in clamshell mode, thereby disabling the internal display. With the hack, the iBook display is off, but it is still logically there, meaning that it is possible to move your mouse pointer to a phantom off-screen position, which I found to be quite a disorienting experience.
Ultimately, the decision on 15" vs. 12" depends on a number of factors, whether you have an external monitor, and whether your application needs differ substantially on the road vs. the desktop.
If you're like me, and my on the road needs typically involve giving presentations, and minor editing of documents, and office productivity applications, and with most of the graphics intensive work done in the office, then the 12" might be a good fit for you. This seems to be a typical experience for students and academics.
If on the other hand, you're a graphics professional who needs to be able to run creative applications on the road, then a 15" or 17" Powerbook would make far more sense.
