There are two versions of the original Dune:
David Lynch's cut, which was the cinema release
Alan Smithee's cut, which is a tv-only release (afaik)
Not sure if one is longer than the other, but Lynch's version is far better than Smithee's, esp if you already know the story roughly.
As for comparing the mini-series and the film heres where I think each did better:
Overall Story: Mini-Series
Scenes: Movie
Sets: Movie
Effects: Mini-Series
Acting: Movie
Costumes: Tie
Edit:
Just found
this article on H2G2 which explains more on the differences between the two film versions