If expandability is important to you, remember that the 15" PowerBooks all have a PC Card slot (PCMCIA), FireWire 800 in addition to FireWire 400, and Gigabit ethernet. The 12" PowerBook lacks a PC Card slot, FW800 and gigabit ethernet. This limits its long-term upgradability and lifespan IMHO. This is in addition to the 15" having a maximum RAM capacity of 2GB, versus 1.25GB for the 12".
As far as the speed difference between the 1GHz 15" and the 1.25GHz 15", check out
www.barefeats.com. They do all kinds of speed tests and comparisons. Also if you can track down a copy, the December 2003 issue of MacWorld (the Australian edition, at least) had a bunch of benchmarks comparing 12", 15" and 17" PowerBooks to each other. Briefly summarised, those results are:
SpeedMark 3.2 Overall Score (700MHz G4 eMac = 100, higher is better):
12" PB G4/1GHz: 142
15" PB G4/1GHz: 138
15" PB G4/1.25GHz: 159
17" PB G4/1.33GHz: 161
Photoshop 7.0.1 Suite (time elapsed, lower is better):
12" PB G4/1GHz: 0:56s
15" PB G4/1GHz: 0:52s
15" PB G4/1.25GHz: 0:43s
17" PB G4/1.33GHz: 0:42s
iMovie 2.1.2 Render (time elapsed, lower is better):
12" PB G4/1GHz: 0:53s
15" PB G4/1GHz: 0:52s
15" PB G4/1.25GHz: 0:43s
17" PB G4/1.33GHz: 0:41s
iTunes MP3 encode (time elapsed, lower is better):
12" PB G4/1GHz: 0:45s
15" PB G4/1GHz: 0:43s
15" PB G4/1.25GHz: 0:35s
17" PB G4/1.33GHz: 0:33s
Quake III frame rate (higher is better):
12" PB G4/1GHz: 64
15" PB G4/1GHz: 73
15" PB G4/1.25GHz: 86
17" PB G4/1.33GHz: 89
Cinema 4D XL 7.303 3D Render (time elapsed, lower is better):
12" PB G4/1GHz: 7:00m
15" PB G4/1GHz: 6:54m
15" PB G4/1.25GHz: 5:33m
17" PB G4/1.33GHz: 5:12m
MPEG-2 encode (time elapsed, lower is better):
12" PB G4/1GHz: 14:45m
15" PB G4/1GHz: 13:59m
15" PB G4/1.25GHz: 12:30m
17" PB G4/1.33GHz: 12:14m
Hope this helps.