You will probably hate to hear it depends, but for an example we have one
environment where we are backing up database table spaces which are pretty
large and it is faster to go directly to a 3590 tape and we have multiple
drives so we can still route client backups to disk.  We have another
environment where we have the same backups go to disk because we are using
8mm tapes.  If you have one client using shmem you will probably be faster
going to disk.  Check your transfer rate in the schedule log and compare it
to going directly to tape.  I definitely would not change until you get a
second drive.