Both are good in their own ways, but Simplex stands out a bit more.
Session is anonymous out of the box due to built-in onion routing (similar to Tor).
Its encryption is also fine, along with other security aspects like metadata protection, etc.
Simplex, out of the box, works through the clearnet, but it can be configured to work only through Tor using a third-party app (Orbot or InviZible Pro).
However, it has significantly stronger encryption, which is quantum-resistant, and it does not use unique IDs for users, providing zero evidential basis for forensic examination.
Therefore, the ideal setup for maximum secure communication is Simplex + Tor.
For Simplex to become the most advanced and best messenger on the planet, it only needs built-in onion routing (like Session).
By the way, Session in the latest updates has bugs that prevent conversations from loading and joining any community on iOS. We are waiting for a fix.