this is a hard one for me
As a self confessed bibliophile, I love printed books...
Especially the ones that are produced with some modicum of quality. I love the whole experience of it, and own a significant number of high end, special edition, leatherbound books.
Physical copies of things are much, much more practical in most scenarios... generally easier to read, browse, etc... eInk has made some headway but not a lot. The general physicality of a book, is a large part of the appeal, presentation, smell, texture, being able to physically turn pages even...
For longevity, a decently produced paper book will last for a very, very long time. From experience, my dad has a great collection of some classic sci-fi novels impossible* to get nowadays, cheap tat paperbacks will last 30-50 years, even assuming stuck on a generic shelf and read every few years! Better editions will last much, much longer "unaided". Paper, naturally, however, doesn't last forever, better quality production, means better longevity, but it's limited.
However, that said, my experience of finding classic, at least out of print, books, has been monumentally more successful digitally than physically. which makes my availability arguments pretty pointless. I'd love to raise the arguments about format, drm, etc. But... if it's stored in plain text, ASCII(Unicode), formatting aside, I'd expect it to be available, whatever medium, for centuries to come!
Generally, I'm arguing for both! An electronic version should definitely be available, but equally validly, the high quality, physically bound versions should also, always be available, for those who want to explore "the old fashioned way" and feel it along with smelling, hearing and... finally, seeing it!
*I say impossible, if you're really dedicated you can find them... there aren't many books i'd pay £100+ for, but there are one or two!