Both format and media are important.

If MDISC meets your requirements for permanency, can you be sure that DVD-ROM and BD-ROM drives will be available when you need them? Will you be able to buy a new one or repair the one you have? Are you sure that you'll be able to read the filesystem? And the files themselves?

I'm neither for nor against this technology or service, but these are questions you should have answers for if permanent storage is your goal.