Almost certainly the 733Mhz G4.
Unless you've done some third-party upgrades, they should not have the same logic boards or be the same G4. The 733Mhz should have a 133Mhz bus and use a newer revision of the G4 spec. Even if this weren't true, the single 733Mhz G4 should outperform the dual 450Mhz in virtually all situations.