This would be a very long talk, but in summary:

- simplex is decentralized (not federated).

- simplex uses more robust encryption.

- simplex focuses on privacy, elements does not.

- Simplex does not depend on DNS.

- in simplex there are no identities and therefore no metadata is generated.

- simplex uses different routes for receiving and sending messages.

They are really two very different tools, one focused on privacy and resilience, the other more focused on group chats.