the architecture would definately be faster, the hardrives faster, the pipelines bigger.
there is the added expandability, the fact that it was a pro machine, not a budget machine.
however, the mini is very small and sleek. the G4 tower... isnt.
(apple a few years ago, at the keynote introducing dual processing, showed a dual processor 500mhz G4 beat a 1ghz Pentium in half the time at a full photoshop job (done using actions), so, by calculation, 2 500mhz G4's were the equivalent of a 2Ghz Pentium. but don't take that as gospel.)