Both are E2EE. Unlike Signal, they also have the benefit of not requiring a phone number, so your account isn’t linked to you that way. In my experience, Session feels more mature, having apps on more platforms and more reliable notifications. However SimpleX has some really nice features, like the ability to have multiple profiles (including hidden profiles).
Ludusavi is also a great option for save backups. It’s not quite as integrated so you have to run it manually, but it does work on other platforms too (like Windows) and is very customizable.