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Both of those computers can run at half CPU speed (DFS) to save power/battery. You could look under System Preferences/Energy Saver to see the settings. In general an SSD should be faster than a mechanical drive for file access. You can also open Activity Monitor to see what the computer is doing. Spotlight indexing has a knack for tying up the computer when you're trying to use it.
 

Xbench had tests for read and write speed of disks.
 
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.
 
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.

Oh yeah, FireWire speed makes a big difference between them.
 
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