Even though I actually I enjoyed the Hobbit films less than the Star Wars prequels and that I think that the Star Wars prequels get more hate than they deserve, I’m going to say the Star Wars prequels.
The reason I say this is that the Hobbit films’ worst offense is just Jackson’s inability to edit himself. He was able to get away with ridiculously long cuts for LotR because, even as long as they were, everything in there served to add to the world building and served a purpose. In the Hobbit films, scenes were extended far beyond what was needed, stuff was added that just didn’t need to be there. But aside from the bloat, and a few CGI messes, the core of the film is still solid. There’s still good characterization, good acting, good dialogue, etc. The way I’d relate it, it’s like the difference between listening to a really long song I love and a short song I love, but played on repeat several times so it’s the same length as the first; LotR is like the first, The Hobbit is like the latter.
For Star Wars, though, it has almost the exact opposite problem. If you look at an overall plot synopsis of the prequels and character arcs, like summarized down to a page or two for each film, it’s solid. The problem is, it needed to be fleshed out, and what it was fleshed out with is just awful. Yes, they look great, and each film has some amazing set-pieces, but the actual storytelling is just bad. Yes, the characters are supposed to be archetypes, that’s what makes Star Wars so appealing, but the archetypal characters in the original trilogy were more than JUST that. So much of the dialogue was terrible. The direction was bad, even though McGregor, Jackson, and Lee turned in decent performances, they were still doing less than I’ve seen them do elsewhere, and the rest turned it irredeemable performances. Worst, the WHOLE point of them is to see the rise and fall of Anakin, and while he has a couple brief moments, considering that his romance with Padme is THE spark that caused him to turn, that it was completely lacking in any chemistry made it just that much more difficult to buy his turn. And that’s why Star Wars is a worse film, because the art of film making just isn’t there. Even if the films were edited down, so many of the important plot points that HAVE to be there are still just poorly done, so you can’t save it in the same way that you can the Hobbit films.
TLDR: Hobbit is better because the art of filmmaking in it is superior and it can largely be fixed by editting. It’s greatest failure is the bloat makes it boring. Star Wars has better set pieces but everything else sucks. That makes it fun in a bad movie kind of way, and with little to edit out that just makes it a set of bad movies.