Here's my two cents.  I have 4 9840 (SCSI) on separate  two SCSI buses in a
9740
Library.  I have not had a problem with this configuration.  These drives
are
fast.  If I had a choice I would go with the FC 9840 drives.  I have a
couple of
point to point SANs and I plan on building a switch fabric SAN in as soon as
I
can get it approved.  The advantage the FC 9840s have is you can connect
directly to a SAN environment.  With the SCSI devices you will need a bridge
(latency lost).  But the main reason is speed at minimum you will get close
to
100Mb transfer rate on a SAN.  You will never come close of getting that on
a
SCSI bus even if it was connect to a SAN.  And LANless backups will knock
you
backup/restore window down.  Since haven't seen the specs. on the FC 9840s I
would think the bottle neck would be on the transfer rate at the tape level
(just a guess)