A G3 is just fine. The fan of the G4 makes noise and is a problem in
reliability. Speed of MorphOS is just the same, I cannot see any
difference.
Speed in Linux is somewhat slower, and MacOS is the same as Linux.
(MacOS graphics is very slow on the G4, too..)
You don't have altivec, which could be nice, if any software would use
it ... is there any ???

BTW; I own PEG1 (G3), 2* Peg2 (G4).

.. make sure, it's really a PEG2, because Peg1 is a lot more freaky to
set up on Linux..