The actual graphics processing does not rely on the GPU, but the Aperture UI does. I have used it on a 1.5 ghz 12" PB with the geforce 5200 (hex edit) and at 1.25ghz 15" PB (don't know the graphics card exactly) and the UI was significantly more responsive on the 15" even though the CPU was slower.
Aperture is pretty much unusable on the 12".
GeForce 5200 in a 12" is not THAT much slower than Radeon 9600 in a 15". Maybe your 15" had more RAM?
MacBook every time.
1. You can run windows applications (if you need to).
2. The processors are faster, much faster. The speed gains will get more and more noticeable as more and more developers move across to universal binaries.
3. They separate the speaker volume and the line out volume (headphones).
4. You can boot from a hard disk connected via USB 2, not just firewire.
5. Built in iSight.
6. Apple Remote / Front Row.
7. Longer battery life, brighter screens and better airport reception.
Far more future proof if you ask me. Although Apple will support the PPC chips for years to come, it seems to be a silly investment.
It really is a black or white decision.
1. If he wants to get a PowerBook, Windows is useless for him.
2. Performance is better only in UB apps that dont rely on GPU
3. Why do you need that?
4. PowerBooks had USB 2 since 2003 when 12 and 17" were released and 15" became aluminium. + PowerBook has FW800 port
5. How often and what for do you actually use that iSight? I bet occasionally and just for fun
6. Even if the OP needs it, it can be enabled using FrontRow enabler. You can use mobile phone or just keyboard to operate it.
7. True, but PowerBook has a 17" 1680x1050 screen, MUCH more appealing casing, backlit keyboard and it doesnt get dirty as much as a MacBook does.