14inch iBook and 12inch PowerBook are very similar albiet their size difference.
iBook
Powerbook
Processor
1.42ghz
1.5ghz
FSB
142mhz
167mhz
L2 Cache
512k
512k
Built-in Mobo RAM
512mb
256mb
Maximum RAM
1.5gb
1.25gb
Video
ATI MOBILITY 9550 32mb
NVIDIA GeForce FX Go5200 64mb
And each model has a plethora of BTO options.
The only real advantage to the 12" PowerBook is the GPU.
I suppose the big question would be, What do you plan on doing with it once you get it?
The iBooks larger screen makes it perfect for Digital editing among other things, however the PowerBook's slightly more powerful processor makes it a solid performer with media editing (iMovie, iDVD, etc.)
If music is your thing, you may consider the iBook with its speakers on the board facing up, rather than speakers placed on the back pointed away from you.
If you plan on gaming, then the PowerBook may best suit you, but you may end up being unhappy with the smaller screen whereas the iBook would solve any size problem.
And also, and this is extremely shallow, but if you want respect for the computer you use, you will want to follow the "bigger is better" rule and get the 14 inch iBook. Sitting in group of people hunched over a little 12inch computer looks funny compared to sitting back at your desk with a 14 inch computer.
Also consider practicality/useability. If you have large wrists/hands you would want the iBook with its larger area for wrist placement rather than risk potential injury with the PowerBooks lack of wrist space.
In the end, whichever you choose, You will end up spending more on the PowerBook than you would with the iBook.
12inch PowerBook with 768mb RAM 60gb HD
(AppleCare)=
$1,574 ($1,923)
14inch iBook with with 768mb RAM 60gb HD
(AppleCare)=
$1,374 ($1,623)
General BTO options...
12inch PowerBook with 1.25gb RAM 100gb HD
(AppleCare)=
$2,074 ($2,423)
14inch iBook with 1.5gb RAM 100gb HD
(AppleCare)=
$1,949 ($2,198)
Hope this helps
