When you do a "Query volume" it will show you volumes that are defined in a
storage pool, regardless of where they are( i.e. in the library or offsite).
A "query libvolume" shows you all the tapes that are physically in the
library and are allocated to the ADSM server (as scratch volumes or volumes
with data on them).

  As for whether to go to tape or disk. If you have only one server to back
up and it is rather large, we would typically put it straight to tape. In
our case , we have 3590 tape drives, and they actually backup the data
faster then going to disk, but I'm not sure about the 7337 drives in your
case.
  The one thing to remember when going directly to tape is that you can only
have as many backups going to tape as you have tape drives. In cases where
we have many servers backing up to an ADSM server, we put them to disk and
them migrate them later, this way we can have many sessions all going at
once to disk.

I hope that makes sense to you. If not, perhaps someone else could explain
it better.
Ben