IO,
I had occasion to research this for a client not long ago - the manufacturers' accelerated testing programs did indicate that WORM CDs have the best chance of retaining data reliably for up to 100 years.
Yes, accepted that's best on testing, not 100 years elapsed time.
The problem with magnetic tape is / was delamination - vast amounts of data (mainly sound archives) on magnetic tape from the 60s and 70s is largely useless now, as the ferric oxide layer has simply come off the backing.
I haven't investigated whether or not this is likely to be a problem with modern tapes, but it scares the crap out of me!
SD