First, what kind of 3580 do you have? LTO1, 2, 3, 4, 5, 6?
What kind of connection does it have: SCSI, fiber?
I would think that a 3573-L2U with two drives would go pretty cheap on the
used market today. We have a 3573-L2U with a single LTO3 scsi drive in
that we have no use for that we're going to toss in on the trade in when
we upgrade from Power 6 to Power 7 NLT next quarter.
A library like that handle duping quite nice. We have a 3573-L4U with
four LTO4 fiber drives in Garrett that did a bang up job duping tapes from
LTO3 to LTO4 and BRMS handled it well.
Then again, some of the other players on the list are pushing VTL
appliances which will give you your local tape for quick restores and then
give you a remote duplication elsewhere. Some have their own software
(which rather scares me). Others simply emulate existing tape libraries
and play well with BRMS.
I would think that the save to tape would be faster than the save to disk
since LTO is no slouch in the speed department and will be using a
different controller than your disk controller.