Digital notes are much more useful for getting long term value out of them, I found. Paper notes are fun, and it's much easier to sketch things freely and just doodle on them.

Paper sketchbooks are also really useful as temporal artifacts. Looking back at them, I know where I was, I discover things I have forgotten, they have different formats, they mix life with work with hobby. I don't think the digital notes will have the same nostalgia factor.