We have an animated video on that here:
https://video.simplifiedprivacy.com/simplexsession/

Session is more decentralized and designed to resist censorship with blockchain based identity.
SimpleX is federated but without having an identity tied to any server.  So this enables you to have no identity.

SimpleX is better for being invisible.  Session is better for this purpose of uncensored free speech. 

 Cool, video, didn't know about your rss feed - subscribed!  

 Thank you for your time