Session has a shitcoin token and uses their own anonimization network instead of tor called lokinet. Besides lokinet's design flaws, the fact that it has a much lower usage compared to Tor gives it a much smaller anon set (crowd to hide inl. To me, Session is the "Brave" of messengers (not in a good way).

Session leaks metadata and your network graph since it reuses the same unique identifier for each user. This is akin to reusing addesses in bitcoin, not very good privacy wise.

SimpleX OTOH has no unique permanent ID, eveything is ephemeral and 1-time use. You can't get spammed just because you joined a group of spammers, since nothing about your account was really leaked by joining that group.

This is all much better explained in simplex.chat