For the HDD route, I'd probably prefer lower density storage, heaps of redundancy, and a good spread of HDD manufacturers.

A quick Google of HDD MTBF suggests that 1 million hours (over a century) is wildly optimistic, and typical failure rate is 2-4% per year, possibly as high as 13%. If e is the failure rate (as a fraction of 1), and assuming a constant and independent failure rate over time, then the survival chances for any one drive are:

    (1 - e)^50

So the chance that any one will fail is:

    1 - (1 - e)^50

With n mirrors (assuming a reliable checksum to verify data in the event of only a single copy of a mirror surviving), the chances of all failing, f, are:

    f = (1 - (1 - e)^50)^n

    log(f) = n log(1 - (1 - e)^50)

    n = log(f) / log(1 - (1 - e)^50)

So, for a reliability of 99.999%, and hoping you can keep the individual yearly failure rate at 3%, so f=0.00001 and e=0.03, n would need to be at least 47.