The reason we chose Metabase was that it had table joins, while Superset doesn't (unless it has added them since I used it). It also looks a bit sleeker. But I strongly prefer Superset; I found that with Metabase I had to turn a lot of things off to make it usable (Let me see "the_table" not "The Table"!), I was constantly annoyed at the opacity around models vs "questions", etc. and every time I wanted to change a question Metabase insisted on creating a new one instead. The real issue here was when we wanted to swap out the data source for a lot of questions but there was no clean way to do so without MB just creating new questions.
Also, Metabase doesn't have serialization unless you pay them AND you self-host, (if I'm self hosting then what exactly am I paying for?) and that's pretty annoying. https://www.metabase.com/docs/latest/installation-and-operat....
But it does let you join tables. Sometimes that's enough to make MB worth dealing with.