If you really don't care which platform, choose AIX.  Unix is Unix, right?
 At least that's what all my "Open Systems" buds tell me all the time.

ADSM is developed on AIX (among others) so new features, performance, etc.,
tends to happen there first.

To answer your question more directly, I, too, have heard more horror
stories about ADSM on HP-UX.  Much fewer on Sun.

Kelly J. Lipp
Storage Solutions Specialists, Inc.
www.storsol.com
lipp AT storsol DOT com
(719)531-5926