I use M-Discs for archival storage, and many Blu-ray burners will burn M-Discs. Of course, I made the mistake of buying 25GB ones, before I bothered to check my "critical" media, which ends up being less than 4.7GB single layer DVD size.
Upon further research, there appear to be contradictory conclusions about "M-Disc labeled Blu-Ray discs." (See below how I still lean towards using M-Disc labeled BD-Rs versus standard writable BD-Rs. You can probably guess which is which in the photo below.)
From what I have gathered,
when it comes to DVDs, M-Discs use a radically different physical material ("stone-like") that is resilient against deterioration and environmental conditions. This is in contrast to standard writable DVDs which use an organic layer and dyes to record data.
However, when it comes to Blu-Ray writable discs (BD-R, BDXL, etc),
M-Discs "supposedly" may not use any material that is inherently superior than the inorganic layers already used in standard Blu-Ray discs. Yet,
there are promising home tests that some people have undertaken that show these
M-Disc labeled Blu-Ray discs do in fact hold up against harsh conditions compared to
standard Blu-Ray discs.
Left: TDK BD-R, 25GB | Right: Verbatim M-Disc, 25GB
Both discs subjected to the same harsh, unprotected conditions for over 6 months outdoors.
Partially buried in soil. Hanging on a branch. Direct sunlight. Rain. Hail. Very high and very low temperatures. High humidity.
With all of that said, I'm most likely going to purchase an external Blu-Ray/M-Disc reader-writer; burn a combination of 25GB (and possibly 50GB or 100GB) M-Discs; label them; and finally store them away in a fire-proof safe,
along with the external reader. The extra cost of the M-Discs seems worth it to me, if the worst-case scenario is they're just as good (and possibly
better) for data longevity, especially considering
it's a one-time purchase of no more than a handful of these discs.
This is not a meant as a backup substitute, nor exclusive archive, of the data in question. It's a one-and-done method
tangential to the ongoing "copy everything over to the next iteration of digital storage" game of Evil Musical Chairs.