Well, here is the backup regime I would recommend.
1) Primary copy: System hard drive
2) DVDs if there are a lot of photos, preferably 8x Taiyo Yuden DVD+R or some AZO Verbatim, these to be used for primary backup and for viewing on other machines from time to time. For smaller amounts, CDs might be better.
3) If you happen to be renting a locker or storage area in a public warehouse, or have another off-site storage option, I would recommend as a third and final backup, for the most important things, the Falcon Century CDs, or if you can't easily get those, Taiyo Yuden CDs. This so that for example in the event of something really bad happening like a house fire or a flood or something, your most important pictures, such as those of family, will be preserved on top-rate media off-site.
CDs are simpler beasts with less physical compression and a larger CRC margin than DVDs plus you lose less in the event of the failure of a single piece. Falcon for example, asserts a 300 year lifespan for its Gold Century range of CDs, but for their DVDs in that range, only :bigsmile: 100 years.
Flash media is best for (semi)frequent use, it's meant for quick rewriting and is based on a series of electrical charges - a very complex one if the device is using MLC flash. And as I mentioned before, they are also prone to sudden and seemingly inexplicable failure.
Optical media, and especially write once media - operates by a system of permanently written, i.e. "burned" pattern of data, and as such has some inherent advantages.
I would say your asbolute best bet for backup/archiving important data is a high quality recordable optical disk.