I'll throw a spanner in the argument...
o You probably need both media types as backup
o Altho the accurate extent depends on the data's importance

One media type is 1 single point of failu
o The DVD drive may produce bad recordings or those only it can read
---- a replacement may have difficulty reading old disks
---- DVD media could go the way of CDR media - variable
o The HD drive may fail, and is a single-instance of the *entire* dataset
---- a single backup of an entire data-set is a lot of risk

Suppose you used just DVD, and you lost the original HD:
o You have 150GB to restore, using 30 disks
---- using & restoring via CDR would be akin to floppies
---- plus if say 1 in 100 disks goes bad, you may find 1-1.5 are bad (*)
o That is going to take say 30hrs to achieve
---- and 30hrs to recreate if the data-set changes regularly

So you may want to consider:
o What part of the data-set is important?
---- data that can be regenerated is ok on HD
---- data that is absolutely critical requires redundant backups (copies)
o How often does the data-set change, or sub-sets of it change?
---- DVD-R gets more expensive the more often change is required
---- DVD-RAM whilst rewriteable & better is more expensive

So the economic answer may be HD for the bulk backup, then spread
the DVD-R cost over several months with incremental backups etc.

Another issue is whether you will backup frequently enough:
o It is very quick & easy to backup a HD to another HD
o It is a very different matter to backup to DVD-R

Whatever backup method you use, verify files regularly.
Not so long ago I had to get data off a CDR, Kodak Gold & TDK
from some years back - quality brands. More recent readers would
not read it - and a laptop drive the least, and even less with heat. It
turned out I could read the data ok, in chunks, from a freezer. As
soon as the CDR warmed up XP did the usual I/O Block Error (7).

The backup media that is most reliable is 2 different types :-)
The backup may not even exist unless you verify it - often.

One reason why companies have used cheap SATA drives.
Appliance - Near-Line storage with integral tape drive
o Faster backup to disk, then dumping out to tape
o Two backup media copies, near-line archiving re fast search/recovery

RAID is not a backup - it is availability :-)
--
Dorothy Bradbury