What would be faster for editing in FCP?

Probably the dual G4.

 

First off, don't even think of the iMac G5 with 512 Mb RAM - that's non-viable for FCP.

Like saying - what's faster, a Volkswagen Golf with 14" wheels or a Ferrari with bicycle tires....

 

look at this thread.... my answer is still the same. The point I was trying to make on the other thread is, video runs at a set speed, 25 or 30 aprox fps If you take in 45min of footage it takes 45 min if you watch a 45 min edit It will take 45min. You could be working on a top end G5 or a 500mhz imac the speed is no different, The difference is what you are doing with it. If you are doing lots of layers / overlays, that when you will need the extra speed. I can do 4 layers of DV on my G5 with out having to render it, On my old G4 I wouldn't have stood a chance.
I would go for the G4 you can have extra internal drives & monitors. You will also need to have a second drive for your media, as it is not recommended that you use your boot drive. with a imac this would be a FW drive. on the G4 this can be internal.