The answer is, of course, "It depends".

For my "big" TSM server (AIX/RS6000), I use any type of disk I can get my
hands on for the disk pool!

We started with SSA, added space gradually by adding large SCSI drives in
every available internal slot.
Also have a SCSI connection to some older STK (actually Clariion) disk in an
external drawer.  No complaints.

First thing I do each night is backup the disk pool to offsite tape.  So the
data is copied whether it migrates out to primary tape, or not.  So the
exposure to data loss is minimal, even if we had a crash of a disk pool
volume before all the data migrated out.

If you are doing nightly backups to diskpool, consider that most of the data
is going to come into the disk pool, then just sit there doing nothing,
until time to migrate out to tape.  So speed for disk pool is not that
important for many sites.

Keep your DB and logs on the SSA, put the backup diskpool on the next
slowest, put the archive diskpool (if you have one) on the next slowest,
etc.  It's the DB and log that are crucial.