Most of the "legit" Apple G3 modules have 1MB of L2 cache, which depending on the configuration of the /350 or /400 G4 chip, may actually cause the G3 to blow the G4 away in those more important (i.e. non-altivec) tasks such as launching apps and rendering pages in firefox.

So if you can, I'd pull up system profiler with each of the chips and find out what they've got in terms of cache. A healthy enough cache in the G3, coupled with the extra few MHz could cause it to be a much better performer.

EDIT: I should say that most of the "legit" Apple /450 and /500MHz G3 modules have 1MB of L2. I know they produced a lot more speeds than those, though.