One downside to the FireWire model is that the VXA3 drive upgrade won't be available for about another year (whereas it's already available for the SCSI model). The upgrade is under $1000, easy to install, uses the same media, and doubles your capacity (serious no-brainer). But, you need the SCSI card on your Xserve to use it.
For the past two years now we have used the Exabyte 1U-10 slot VX Packetloader (Firewire) version with our XServe installations and have been very pleased with it's performance. I would highly recommend this drive and you should be able to buy it with Retrospect bundled. It is a solid backup solution. The price has also dropped substantially (at least here in the UK, it has) which has made it more affordable for Xserve users looking for an automatic backup solution.
I have been using the Exabyte VXA-2 for about 2 years as well and have had it hooked up to both my xServes(one is a G4 and the other G5) via firewire. I have been really happy with it as a back up solution. I have backed up directly to tape with it and was happy with the speed and I am not staging everyone to an xServe RAID and then backing that up to tape and the speed has been just as good.
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How to connect an Exabyte VXA-2 PacketLoader 1x10 1U Tape Autoloader
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