IBM marketing has always done a great job at preventing "side-by-side"
comparisons on their specifications sheets.  They often list different
performance specifications.
As far as I can tell, the IBM 3580 Ultrium drive holds an impressive 100GB
native (2:1 compression) whereas the IBM 3590-E1A can only hold 40GB native
with the extended cartridges (3:1 compression).  IBM also publishes a
sustained data transfer rate of 15MB/second on the Ultrium, making it faster
than the 3590-E1A's "maximum drive data rate" of 14MB/second.

Having said that...  my IBM storage representative says that the 3494
library is a more industrial tape library intended for larger enterprise
environments than the Ultrium 3584 tape library.  IBM offers and supports
extended utilities for the 3494, such as StorWatch Expert software.  Also,
the 3494 library and 3590-E1A tape drives can be FC-AL attached.  Currently,
the Ultrium 3580s can not.

But, I must admit that for the GB/sq. foot the LTO 3584 is very nice.  I've
worked with both libraries and they are both among the top in the industry.


Here are the specifications links...

http://www.storage.ibm.com/hardsoft/tape/3494/prod_data/g225-6601.html

http://www.storage.ibm.com/hardsoft/tape/3584/prod_data/g225-6853.html

Hope that helps!

Regards,

Chris Neumeyer