Rian is a better filmmaker because he is passionate and JJ is only in the business due to his heritage and as part of the corporate machine. He's a fan, writes like a fan and is good at what he does but never challenges himself on his ambition.
However the elephant in the room here is Star Wars. The normal rules don't apply here, as both directors have done slightly different than they usually do, in a sense based on just how their movies in Star Wars were.
Rian underdevelivered. I'm not a fan of his movie Looper. It's too edgy and twisty as a concept for me, but that's the kind of experimental films you sometimes make. It's like Christopher Nolan and his pursuit of bendy ideas rather than just thinking about which emotional drive is the best story to tell and therefore best film concept to make. So I'm not a fan of Rian but he seems like he has a style and a skillset that works for him. Well, regardless of his own biography that skillset didn't pay off for Star Wars. The Last Jedi is like Mass Effect 3. It simultaneously contains the best AND worst moments of the brand all by itself. As much as it gets away with daring and sometimes experimental ideas, it took some things too far and without thinking it through and just leaves you unsure or confused. It has a great theme about what it means to become a Jedi and how we all have a potential to resonate with the Force. It delivers its message as Rey learns she has no important background and as a child picks up a lightsaber, and then it has Luke who's force-projecting planets away, a provocatively un-Star-Wars set with Snoke dressed up like he's part of that music video, and is shortly killed because that was kinda bad. It had space mommy that they waste way too much time on, they unceremoneously kill Phasma and they send Finn on a journey to a prequel-like casino and naive romance about horses. The movie was a mess tonally. It contains scenes that are interesting for Star Wars but it also contains scenes that feel like they're designed to shock a Star Wars with little other reason than to see the rise it would get out of it. It seems reactionary to the business behind Star Wars and the lameness of JJ's premise. It's a destructive movie that bangs on a drum with mighty force, makes you cover your ears.
And JJ is just boring. He's not bad, just boring, but his movies hit a perfectly accessible middle road of entertainment and inoffensive fanservice and fanfiction. It's Star Wars alright, but the only thing that you can complain about is that it takes absolutely 0 risks and then it has the flaws of a JJ direction, which is that every scene must be desperate, closeups of emotional faces must be had, cameras have to shake and you can't have a scene in which the protagonist isn't running away from something with a camera panning out to show the impressive scale of its set-piece.
I think Rian's movie was kinda pathetic, I'm just gonna be straight up with that. It's critiqued by nerds as lore-breaking, franchise-ruining, leftist-ridden, whatever. It's lauded by supporters as a "subversive" and masterpiece story. I felt neither, to me it's just an attempt to shock fans, to challenge JJ's poor creativity, Star Wars's noteriety as a toy-brand (by killing Phasma), and as a subversive film to the lore and history of Star Wars it's a mixed bag. There's some scenes that were genuinely going somewhere, but then it was chopped in half by the "truth" behind Luke's story about Ben and has a rather self-serving framework for the whole "first order" backstory by the end. It kills off the many things it's supposed to drive home. It's pathetic to me because it ends up prematurely chopping off lingering threads in the middle of the act 2 it's supposed to be without replacing it with something constructive. It's so easy to look at JJ's foundation and say "I don't like it, I'll tear it in half, now what do you think?" and many fans were thinking the same thing. Kill that boring-ass OG-repetition already... but don't just kill it, correct it. TLJ doesn't correct it, it just kills it.
So regardless of what I think of Rian and JJ, I have to look at what they specifically did with Star Wars and say, they could've done a better job, and that's on them both, but in some ways especially Rian because JJ's framework wasn't lacking in function, direction or motivation. It was just middle of the road barebones entertainment.