Unless it's really important for the drive to move from machine to machine, you're much better off getting a second internal serial ATA drive. The latency introduced by a FireWire bus can interfere with some audio software. I don't use Digi, so I can't say anything about that in particular, but in general, internal ATA will give much better performance, if only because it doesn't have to go through all the SBP2 to ATA translation in the firewire<-->ATA bus bridge.
Of course, if you already have two internal drives... I would suggest using a Granite Digital case. Good performance and they're good about maintaining firmware updates for their bridges. Just my $0.02.