The main issue with HDDs is that even though they are cheaper on the whole, they are much riskier. If a DVD gets too scratched or the dye gets messed up, at most you will lose 4 GB. If OTOH your HDD goes, depending on the the way it fails it may take a 50-100x the amount of data with it. And HDDs by their very nature are much more fragile then even the cheapest optical media. Strong mag fields, too much shock, too much humidity/heat/cold, even an electrical surge can all nuke an HDD much easier then an optical disk. I have actually lost one because i left it in a drawer somewhere and spiders made a home in it LOL.

Plus you can get inexpensive disks that are good quality also. Just get a few named brands and find which ones work best for you. Then run those through a tool like DVDIdentifier which will tell you which media is actually under the brandname. Some companies rebrand media and mark up the price, so you can often get that actual media for cheaper then the branded one with the same quality.

DVD Identifier:

http://dvd.identifier.cdfreaks.com/