To me, it looks like supabase is designed to take full advantage of postgres features. plpgsql triggers + RLS + clientside auth + streaming changes to subscribers (including via web hooks) are my favorite features. (They also have js edge functions, but I use lambda instead b/c I prefer python)
Supabase feels like the scrappy company with amazing focus, akin to an early MailChimp (circa 2007). PlanetBase feels more like early Snowflake - massive scale, focus on performance, can match anything feature-by-feature. One is a master of their craft, the other is a gorilla at scale.
Curious what others think. I haven't used PlanetBase extensively so don't have much to go on except their marketing.