Single 1.25gHz, hands down.
If you are going with Apple's specs, the dual 450 has:
Bus Speed: 100 Mhz
Data Path: 64 bit
RAM Type: PC100
Mininum RAM Speed: 133 Mhz
Maximum RAM: 2 GB
Level 1 Cache: 32 k data, 32k instruction
Level 2 Cache: 1 MB backside, 1:2
Video Card: ATI RAGE 128 Pro
VRAM: 16 MB
==========================
Whereas the 1.25 has:
Bus Speed: 166 Mhz
Data Path: 64 bit
ROM: 1 MB ROM + 3 MB toolbox ROM loaded into RAM
RAM Type: PC2700 DDR
Mininum RAM Speed: 333 Mhz
Level 1 Cache: 32 k data, 32k instruction
Level 2 Cache: 256 kb on-chip, 1:1
Level 3 Cache: 1 MB DDR SDRAM per-processor, 1:4 (2 MB for 2x 1.25 Ghz)
Video Card: ATI Radeon 9000 Pro
VRAM: 64 MB
==========================
Thats a significant difference, and not just because of the processor. FSB and type of ram is very important. Besides, I dont think the 450 and the 1.25 CAN have the same video card, and I know it cant take the same type of ram. The single 1.25 is a better choice, not just because of the processor speed, but the architecture of the box as well.
Dont forget, a dual machine doesnt mean it runs twice as FAST, it can just do threaded tasks simultaneously.