Audio CD and video DVD are completely different supports, and completely different are also data stored in them, so it is a nonsense to do a comparison
Bear in mind that there are also a lot of myths about audio CD ripping, and most of them are totally false :doh:
My suggestion for backup is to keep the originals in a safe place untouched, and for everyday usage make a copy on DVD (single or double layer as you prefer) or on external HDDs as you already do. When the copy is not more usable, you can make another from the original.
If you are concerned that also the pressed disc can become damaged in some way, you can also make a backup of the original disc with a trick to improve safety. I suggest to rip the original disc as ISO file on HDD, then make a RAR archive of that ISO file splitting it into 1GB chunks. Very important: when creating the archive, be sure to add parity data on archive (I like a lot WinRAR for this feature). Then store these chuncks on a couple of DVDs or on HDD.
If you need to restore original data, is it sufficient to extract the ISO file from archive