Oversimplified most apps will run about twice as fast on the G4 as on the G3.

The big problem of the G3 is the limited memory throughput.
The memory throughput of the G3 is the the range of a 68K CPU.

On Linux and MacOS were applications tend to be much bigger than on MorphOS, this memory limitation is slowing the G3 down a lot.

On MOS the difference is not so visible as most apps are small and as the G4 memory throughput is limited by MOS atm anyway.

The G3 has a memory throughput similar to a K6 or 68040.
Linux will run on the G3 in comaparable speed. If you want to use Linux a lot then better get a G4.

Cheers
Gunnar