Well, to be honest, there are not that much differences in speed, if you compare some of the faster G4 machines with a modest G5. Due to the faster memory access, the G5 is able to play 720p videos properly, which won't work on any G4 machine without heavy frameskipping.

A G4 Mac mini only can have 1 GB of RAM, while the G5 can use upto 1,76 GB. MorphOS and its applications have a little memory footprint, but this makes a difference when doing memory demanding tasks like browsing with OWB or Wayfarer.

Oh, and it's easier to upgrade the harddisk of the G5, as you can use SATA drives without any adapter. On a Mac mini you are limited to PATA drives (or you are using mSATA drives with a PATA adapter).