CPU speed will benchmark practically the same between 1.5GHz models, with the 15-inch generally having a slight edge.
Outside of CPU speed, the 15-inch models outspec the 12-inch in terms of;
1. Max memory config
2. GPU performance and capabilities
3. Ethernet speed
4. Firewire speed
5. Display resolution and aspect-ratio
6. Heat dispersion / cooling
Despite these points, and having owned all of the PowerBook G4 models, I prefer the 12-inch form factor, and would go to this to tinker instead of a 15-inch unit, but that's just me.