I would suggest that using DVD+R DL media is far more costly than other available options. For starters, the only disc that has consistently proven itself reliable is the Mitsubishi-made Verbatim DVD+R DL
. Other brands like Memorex, TDK, Sony, etc., have largely proven themselves to be unreliable (or to be outright crap). Most of those other discs are RITEK DVD+R DL discs or CMC DVD+R DL discs, although there are some even worse ones under no-name brands. (Playo's UMEDISC, for example.)
So your choices on double-layer/dual-layer media is more limited than single-layer DVDs. You'll pay more for Verbatim's higher quality product -- about twice as much as the low-grade stuff. And that's fine, if you need DVD+R DL discs that are reliable.
It's really not a huge increase in file size -- it's just under half (about a 40-45% gain). The next largest optical disc is a 25GB or 50GB Blu-ray BD-R blank.
However, I don't really suggest either DVD+R DL or BD-R for archival use. For masters, yes. But a "master" is the first copy of something made from source files that are archived.
ARCHIVES > MASTERS > DUPLICATES (DISTRIBUTION)
The problem with 8.5GB burned DVDs, and 50GB burned BD-R's is the second layer. It's suspect for long-term accessibility. The way optical media works is that light is bounced off a metal foil, and it picks up the lands/pits (1's/0's) in that process. The second layer of a pressed disc is simply twisted off-axis with the first layer. For a burn disc, the laser has to permeate a semi-transparent (translucent) layer spacer, as well as pass through the first layer dye. Readability on the second layer is always weaker than the first. Right now, that's not an issue. But as laser hardware ages (and becomes more obsolete), and as the discs age, that slightly harder-to-read layer could become too hard to read at all. Therefore, data would be "lost". Being able to access the second layer reliably is what halted the existence of recordable dual-layer media to being with (and at one time, engineers could be quoted as saying it wasn't possible to do). Indeed, rewritable DVD-RW DL and DVD+RW DL don't exist beyond lab samples because it's not reliable (as well as too costly).
The issue with Blu-ray is furthered by its low hover to the laser assembly, which can cause contact with the disc. This can cause physical imperfect on the disc -- even at a microscopic size -- thus hindering quality readability. And sometimes a few micro-abrasions or micro-scratches can mean the difference between a disc that reads and a disc that doesn't.
Given the proprietary nature of Blu-ray (Sony), and the history of Sony-backed products in market, the longevity of the format is also suspect. It may die away quickly like their Betamax video format, or a long line of other audio/video products.
Good archive policy suggests using both a magnetic and an optical format, anyway. And in duplicates, stored in various locations.
A hard drive is a fine way to back up data. And then duplication that hard drive, and store the copy at that home of your parents/kids, or your office desk drawer.
Same for DVDs. Burn a mix of discs, including both DVD-R and DVD+R, from reliable brands like Taiyo Yuden and Verbatim (Mitsubishi DataLife), and spread the copies around your sites.
For backup drives, I suggest using an external unit with a power button. Mainstream brands like Western Digital and Seagate provide those "smart on" drives that turn on with the computer. For some drives, I like that. For others, I hate it. In the case of a backup, I hate it. While I could unplug the drive, I'd rather use a power switch. For that to happen, you'll generally need to build your own, using a good Seagate or Western Digital SATA drive, and an external enclosure.
For these parts, I suggest shopping from Amazon
. For ready-to-go "smart on" drives, look for deals from Buy.com
(they had a 1TB for $69 shipped last week). If you can assemble Legos, and know to not put round pegs in square holes, you can build an external drive. It's literally 2-3 wires, a couple of screws, and some pieces that slide together. A monkey could do it.
There's really no more danger to a drive "wearing out" than a disc being ruined by accidentally dropping it. But do consider turning it off when not in use -- just in case.
There are also some RAID 1 mirror drives -- eSATA, Firewire 400, Firewire 800, and USB2 -- for on-site instant duping of data. It protects against drive loss (hardware fail), though not data corruption (user/software fail). For more on that, read this thread on best RAID hard drives for computer backups
Hope that helps.