I would caution against the small MO discs (Mini-Discs). So many manufacturers are making them now (and sub-contracting their manufacture to the lowest tender) that there are real quality control issues. I have Mini-Discs which just fail completely, or sometimes fail on one Mini-Disc reader but not on another. That's okay for music compilations, but I wouldn't trust them for data/archive.

The biggest problem with CDs is that you are likely to scratch the surface, sooner or later. (somebody-or-other's law states that will happen, no matter how carful you are.) Scientific papers have discussed molecular migration (filling in of the CD's pits over decades of time basically) and also the 'wrong ink' on the top surface chemically affecting the data-carrying layers beneath.

The other consideration is error correction. Our music CDs (many being now 12 years old) may be deteriorating, but error-correction is being applied and our ears can't notice the degredation of quality because it's easy to correct the audio. A 12 year old data CD may not survive as well !

Maybe the answer is to burn replacement CDs every 5 years – a time-consuming option.