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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.
 
Agreed. Although Tiger (and I'd assume Leopard) take advantage of multi-processors, the advantage is minimal. The only real benefits to dual processors is the lack of processor bottlenecking, they don't run faster, just wider. ;)
 
Regarding "snappiness" of your system (Finder, Mail, Safari etc.) the Single 733 will be faster.
If you convert QuickTime movies from DV to H.264 the Dual 450 will have done the job in less time...
 
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.

Yeah, I know that they aren't the same logic boards!
 
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