They both have S and Mg, but Langbeinite also has K. Langbeinite is more potent too.
I have used both, and Epsom is definitely milder.
I've noticed sometimes when I use Langbeinite or the liquid Potassium Sulfate that it can actually make my K deficiency worse. I'm thinking it might be from S toxicity, but haven't fully figured it out yet. I know elemental sulfur will lower soil pH, so maybe that has something to do with it. I'm trying to figure that shit out, but there's not much to find on sulfur toxicity.
Edit: I almost mentioned what Holla said, and I totally agree with the Epsom comment. He worded it almost perfectly. But he's talking about salt based nutes, and not organic liquids.