Thanks for this interesting question. I'm on the side of those who believe that hard copy books are the best way of preserving family histories.
I wrote a book on my mother's family history and privately published 50 copies. I sent these to every relative I could think of who might be interested, so copies are scattered throughout members of the family tree. I would like to think that these will be passed down to future generations.
One other option to consider is that for a few hundred dollars extra I could have had the book included in the Amazon catalogue where it would in theory remain available for purchase indefinitely.
As somone who has spent the last 15 years producing digital copies of out of print books and records for the family history market and archives I would say the best way, without doubt, of preserving family histories is in book form.
Books can withstand a tremendous amount of abuse and still be readable unlike digital records which may fail at the slightest thing.
The major downside of printed books is cost, the cost of printing, binding, etc. and subsequently, for readers the costs of purchasing a copy, including postage.
Having said that those costs should be weighed against the equipment required to access a digital copy and perhaps justified that way.
Cheers
Guy