Better to rewatch the old version before the new version? Or just watch the new one? It's been 20+ years since I've seen the old one.
Now that the sequel has been approved I wouldn't watch original or read the book until you watch both of these movies (then if you want to geek out, geek out).
I'm a bit of a Dune nut and I did rewatch the 84 movie a few months ago. For people bagging on the original and championing this new one...I think the new movie actually strongly validates the old one in lots of ways. It takes waaaaaaay more visual queues from 84 movie than the Jodorowsky production sketches. The costume and set design of the original Dune are among the greatest in cinema history, there is some good stuff. Some people hate Lynch's weird horror-inspired additions but really that's like less than 5% of the original movie. Real weakness of the original was the story was just too long for one good movie.
I love the Jodorowsky's Dune documentary (as a creative professional I have been there with cancelled projects), but it has created a bit of a bashing subculture against the Lynch movie when I think that movie had actually been growing in cult status for quite a while. IMHO 84 Dune was a bomb because they pushed it as the next Star Wars and it's nothing like Star Wars, not that they completely destroyed the subject matter, had bad acting, or bad film making skills.