It's been a while since I watched those minis (In fact, I don't think I've seen them since the second miniseries aired), but I did enjoy them a great deal at the time, as I remember. I felt like someone (in this case John Harrison) had finally done the book justice, and I was ok with the changes that they made, especially in the adaptation of Dune: Messiah and Children of Dune, as it made sense for TV purposes.
But the Villanueve adaptation captures most of the elements from the book that I've seen. But what's best is that a sense of mounting tension is very well textured in over the course of the film, and its something that didn't seem to be done as well in the mini from 20 years ago. I find the new film to be very haunting.
So the old mini is probably worth a watch, although I would probably want to wait until part 2's release for a true comparison.
Not following the industry news, I thought they'd have been filming them right after each other.
Legendary films, for whatever reason, opted only to do one at a time. Oddly enough, it kind of works out, as the second part of the book takes place three years after the first.