The 3494 with the 3590s is great for a lot of stop start writing.  It is
also intelligent so it does a lot of the managing of the tapes.  On the
downside that adds a layer of complication.  It is also more expensive.  The
3580 is great for streaming data, moves fast, and isn't as complicated.  Is
also isn't as smart and doesn't so as well with the stop start type writing.
For us at least the LTO tapes are larger so we had 20 tapes that reduced to
6 which was wonderful.

Hope this helps
Becky