Tbf signal is pretty good. Since I‘ve had someone else mention signal in this thread before I‘ve just spent an hour reading. Here‘s what I‘ve found:

the signal protocol and app

The signal protocol is rather secure and anonymous, trying to achieve zero knowledge situations with the server owners. Your contact list can apparently be reconstructed by following the messages, not sure how that works. Just found this in a reddit thread

xmpp (omemo)

The protocol used by whatsapp. Is also very secure, some vulnerabilities have been noted and some concerns with the ability to break the encryption. Not sure how that would work either but I‘ve read that a couple times now.

the matrix protocol

Matrix is notably more focused on stability and has more decentralization capabilities (apparently easier horizontal scaling since your chats are saved on ever server your chat members are on) therefore less failure likelihood.

summary

All the protocols are vulnerable to some attacks and none has a significant advantage. The xmpp foundation has apparently been influenced by google at some point and the matrix foundation has thinly veiled corporate ties which may or may not become a problem. All in all, it’s probably not important which messenger you promote, I have to say the interoperability of matrix through web api bridges has been noted by multiple sources.

Have a good one. :)