It would be very interesting if someone could find papers with statistical research proving what is safer for a period of time greater than 10 years. A human brain or an HD. Something like this would definitely be very useful for many future applications. The simplest way to think about this is that it is much riskier to store a password in the brain than in a hard drive.
I doubt such papers exist.
The answer to the question depends on repetition, though. You can commit a 12 word phrase to memory relatively easily, probably in under an hour. You'll remember it for a day or two, no problem, but you'll have forgotten most of it in a week or two unless you practice it a few times every day. There's no way most people will remember something like that for >10 years unless you are repeating it at least a couple of times a week.
And then, as I mentioned, all you need is a mild blow to the head or a bad infection and you've forgotten it.
450+ tx ins without a single tx out 
