Drop a CD... now drop a hard drive!
Written optical media is likely to "fade" over time - a good write on good media will last longer than a bad write on poor media. But it takes some actual physical damage (a hard-floor drop can crack / chip or seperate layers, but most optical media should survive a carpet drop).
On a HD, data fade is unlikely during the realistic working life of the drive, however, it is vulnerable to shock, static, magnets, and electronic/mechanical faults.
HD backup will beat optical media, until the HD fails.
Other than magnet vulnerability, there's a lot to be said for tape backup, a tried and tested technology where the limitations and vulnerabilities are much better understood.
A BACKUP, suggests that you still have the original, and that you should be making new backups on an appropriate cycle - the backup has to be reliable for the time between, lets say TWO backups.
An ARCHIVE, suggests that you are putting away material for some time, and may NOT have the original copy available. So the demands are greater - Archives should be duplicated initially (preferably on different media in case of media batch problems), and should be reverified at suitable intervals, generally copying onto current media as well and adding to safe storage).