Stormlight is better in every respect. The story is not just bigger and grander, the writing is also superior, period. The POV characters are more fleshed out than Vin. Mistborn also suffered from one inherent flaw. It's structure was inherited from the first book, which was about a group of thieves planning a heist, and that worked great. But then Sanderson had to apply that to 2 and 3 and it made less sense. Stormlight is more consistent. It also encourages rereads. Lots of Interlude chapters only make sense if you go back to them because they introduce characters who only start to matter to the story later, sometimes two books later. It is very well planned out. The characters are also better. Shallan is the Vin-like teenage girl Sanderson is fond of writing, and both of them are pretty good, but then you also have a bunch of other POV characters with strong arcs and personalities. Maybe Firstborn has the best secondary characters overall ; there are at least three strong contenders in the crew. That's because there are so many POV characters in Mistborn however, which decreases the pool size.
The only thing I cannot comment on is the overarching arc, because we're not even 50% in in Stormlight. I don't find Mistborn as subversive as some. It doesn't really upend the rules of the genre in my eyes. Stormlight so far is even more cookie cutter though, even if not in a bad way. There are a couple of reveals here and there, but we've basically always known who the baddies were, and it appears Odium is the ultimate Cosmere villain, so it is unlikely there will be many suprises there. It will end up being another Objectively Good Guys vs. Objectively Evil God of Evil type story.
I have other gripes with Stormlight, but comparatively, those are the points that matter.