It's not clear from a quick glance, Mistborn and The Way of Kings take place in the same universe?
Stormlight Archive is 10 books in two 5 book arcs. 4th book is out now, 5th book will be out in 3 years. So probably a decent time to get into Stormlight. (Sanderson makes a schedule for each of his projects and has an incredible talent for hitting his targets.)
Mistborn is a complete trilogy although he continues to publish other novels set in the same world.
Way of Kings is ongoing. It's on book four now. Each book is over a thousand pages so there's a lot there. I don't think it's a problem to start. Books 1-3 are great and stand alone pretty well. It's started to drag with book 4 in my opinion. Like so many other huge epic fantasies, it has too many characters, too many plotlines, too huge of a world, and it's difficult to maintain the epic feel with all that sprawl. I'm worried for book 5.
> Mistborn and The Way of Kings take place in the same universe?
They take place in the same universe (literally) but they are on different worlds. So they don't have anything (much?) to do with each other (yet?)
Book 4 of Stormlight does have some pretty big references to the original Mistborn trilogy, too.
On the whole I try to read books in the order I bought them (ish), but Sanderson is one of the authors I'll just drop everything for when a new book comes out. Disclaimer: he does have some bad habits (mainly inserting "wise ass" characters who don't fit the tone or setting, and who I strongly suspect carry the author's voice a little _too_ directly). But he does epic world building incredibly well, and very different to just about any other author I've read. He also writes action exceptionally well.
Mistborn and Way of Kings are in the same universe but separate worlds. I don't want to spoil too much but there is some crossover involved.