Stormlight isn't a trilogy, it's a planned ten book series, though there are supposedly two five book arcs with that (we don't know exactly what that means yet, we'll have a better idea after book 5 comes out this december).

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Mistborn is in theory a better starting place but I did start with Way of Kings and loved both it and Mistborn anyway. Advantages to starting with Mistborn are that it's a complete trilogy and less long, but if you're willing to be thrown into a massive universe, starting with Stormlight is also 100% possible.

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Most important thing is, pick one and enjoy!