[Veratyr]

XMPP could be a fantastic option but it's missing push notifications, which are an absolute killer these days. You either don't use XMPP on mobile or take a massive hit to your battery.

ChatSecure has a nice writeup about this: https://chatsecure.org/blog/fixing-the-xmpp-push-problem/

[edhelas]

Conversations have Push Notification support https://github.com/siacs/Conversations/issues/1171. There is a standard and it's implemented in some XMPP servers as well.