I used to help sell both as a sales guys techie.  IMHO:

Veritas NBU needs more tape drives as we did backups to tape, but it was
good to allow multiplexing data onto the tapes.  You really MUST keep enough
data comming during your backup periods to keep you tape drives streaming or
your backups slow down.

TSM 'incremental forever' is its big selling point and having big disk
pools.  Keeps down the amount of data you backup.  This is a big deal for
'over the network' kinds of backups.

Both have a database.  You can re-build a NBU database from the tapes in a
disaster situation, but you must re-catalog the tapes to build a new
database.  TSM doesn't seem to function from previously generated backups
unless you have a database backup available (someone please correct me on
that).

In selling them, TSM takes more of a server (CPU and disk), NBU takes more
tape drives.  Scheduling for NBU can be a little trickey to balance
fulls and incrimentals properly for the I/O and bandwidth available.

Software cost seems to be similar if I remember right.  Both vendors will
fight for a large account, or to knock the other out of the running if
needed.  But that is partly the VAR rather than the base vendor (Veritas or
IBM).

TSM was a harder sell because the 'never do another full backup' just
doesn't ring right in lots of folks ears.  So they are pre-disposed to NBU.

The NBU Media server is a neat thought, and allows lots of backups to be
managed centrally.  Similar to clustering TSM servers, but not the same
concept.  I do not remember how using media servers drove pricing.

As a techie working with a sales guy, my biggest thing was to get the I/O
properly balanced between client, from the cleint end bandwidth, server
bandwidth, bandwidth to tapes, enough CPU to  drive it all, and disk space
for the databases (roughtly the same).

I didn't like doing TSM with fewer than 4 drives, but NBU could run well on
one if you don't have enough backups to need several drives.  A library of
some kind (at least a tape changer) was needed for NBU to work efficiently.

Once you got a lot of backups to do, TSM seems to scale better to me since
you don't have to have a tape drive available during a backup (but it is a
good idea :)

TSM uses tapes more wisely (reclaimation et al), and needed a larger library
so it could handle the tapes wisely.  --- I had no prefernce when selling,
as they address slightly different needs.  But as a customer, I prefer TSM,
because I like to feel that the computer works for me, I don't work for it
:)
I also do a lot of over the network backups and only have two drives
currently (more on the way :)

Neither solution is 'cheap'.  For some other solutions may be a good answer,
like: Amanda (uses a disk pool, but does fulls and incrementals), Veritas
BackupEXEC (little brother of NetBackup but doesn't come from the same gene
pool as far as I can tell.  Another full+incremental type.), Arkadia (I know
nothing about), home grown scripts work too (but for an enterprise, may not
be a good solution for lots of reasons).

For my cut, TSM once set up was a lot more 'hands off' than NBU was.  But
that is just from my experience and preferences.  YMMV.