Depends heavily on your use case, but there are a lot of advanced features.
From a review last year (so some of this may be outdated):
- Broader datasource support - 20 sources in Superset, vs 12 in Metabase
Superset supports:
- view creation by users ('create table as'), as well as materialization
- geo/GIS based visualizations
- day over day, week over week, etc. charting; moving averages, etc.
Superset is missing data catalog-like functions, which metabase has, though I didn't evaluate metabase's data catalog in depth. It also requires you to write SQL, where Metabase includes a query builder UI - though there was a rather strange difference in capabilities within Metabase between the SQL-based charting and the query builder UI.