http://privacyy3tsy4mge4qmg4nsid2vnhl7szzupphhkfsxvayx5tl2ztbqd.onion/briar-1-private-messenger-but-no-audio-calls/index.html
The big difference between them is that Briar is purely peer-to-peer and so it requires the other person to be online to receive the message. While Briar is peer-to-peer, these messages can be first routed through Tor or in-person bluetooth. On the other hand, Session stores messages on a decentralized database called the Swarm, which is coordinated by blockchain incentives to enable you to receive the message anytime.