...
>The standalone library, while cheaper, means that in 2-3 years I will have
>filled up yet  another library now destined to collect dust in a corner.
...

Not necessarily.  The Ultrium roadmap:
      http://www.lto-technology.com/newsite/html/format_roadmap.html
      http://www.qualstar.com/146252.htm
talks of doubling capacity every generation, which could mean just swapping out
drives (which could be dispositioned by inserting into stand-along cabinets for
local system use, or sell on the used market).
It may depend upon your timeframe relative to LTO evolution, and the vendor's
inclination to allow the next generation to fit into the old drive positions.
(A big hint to your IBM rep could help keep this true.)

IBM has been wonderful about evolving existing libraries and retaining cartridge
dimensions, making a long-term investment very economical.

  Richard Sims