I'd use the Mac, mainly because a 1200 would be wasted for such task and the Mac might have higher disk access speed than the miggy. Make some tests on both though, see if you can all three stream at the same time from the Amiga. If the PPC Mac has IDE, that's a plus since you can have rediculus amounts of storage for nothing (as opposed on using SCSI disks). Then of course SCSI is known to be more reliable for such tasks.
Slap a UNIX system in there, set up NFS and off you go. (Or samba if you have windows machines in your network).
