I have 3.9 installed mainly to use larger drives on machines with fast scsi controllers and at least a 68040, though this can be done on 3.1 w/ TD64. On anything less than a 68040, 3.1 is fine and you can do everything 3.9 lets you do without bogging down. This is especially true if you have a machine w/ 3.0 roms and don't want to upgrade, you can still get rtg, etc. w/o 3.9.
