Books

honestly, mistborn isn't very good and there is no reason you can't just immediately jump on stormlight. yes, there are tons of references and characters which does make stormlight awesome because you'll go "omg no way! it's HIM!", but it isn't strictly necessary because stormlight doesn't actually assume you've read the previous cosmere books (few exceptions where brandon just teases too much it takes away from the plot, i.e. the mysterious scarf wearing woman. yes, we get it, it's the princess from warbreaker, and she's looking for one of the gods from warbreaker, that we're also familiar with, and the god in turn is looking for his weapon). but ultimately these characters have their own story arcs and most of them aren't immediately important to the plot and more like easter eggs.

now, i should point out that ... i really don't care for mistborn. and for now, it doesn't matter. but after the next stormlight book, there will be an in-world timeskip, and then another 3-6 mistborn books, and then both the mistborn series and the stormlight series become one joint series.

but there is really no reason to read stormlight before mistborn in its current state because the only real way you'll know for sure one of the hidden characters in stormlight is a particularly well loved character from mistborn is if you read the wiki. there is also a guest appearance of another character in i think the first book but it doesn't make sense in the timeline and it's inconsequential to the plot in every way.

I really enjoyed the first 3 Mistborn, though I'll admit the third was the weakest of them. I'm not sure why you need to have read them for the other stuff, though. I've enjoyed a lot of his works, but the magic systems are quite different between them, despite apparently being in the same universe. (Which I didn't realize until I'd read a lot of his books.)