We used both, and ended up going with Superset for internal dashboards and data exploration. Redash is more polished to look at, but Superset offers more functionality. However, some things are still rough around the edges; I get occasional 500's during normal navigation, and the filtering options are immature, using only "IN" or "NOT IN" (you have to say things like "field not in FALSE", etc.)

Still, though, quite powerful, and we hope to push some improvements upstream in relation to my comments here.