Hi Paul!
I don't know anything about AIT. We are currently using 3570 because of it's
fast data access times, but we need a larger library. Since we are already
using the largest one (3575-L32) our only option was to use more libraries.
Not something you would like to do in a TSM configuration.
We narrowed the choices down to two, 3590 and LTO. 3590 because it's
compatible with our OS/390 environment, should we want to integrate in the
future, and LTO  because it's an open architecture. We choose for 3590
because little was known about LTO about a year ago.
Kindest regards,
Eric van Loon