I think Matrix sucks honestly. Issues with encryption making messages unreadable, outdated clients that all have or lack different features, extremely slow development, not prioritizing features that would pull users from apps like Discord and make it a more attractive platform for the average person.
Encryption is cool and good and necessary, but Matrix will never be mainstream and even something relatively new like Fluxer, which is an unencrypted and open source Discord clone, is going to be more popular once it gets out of beta if it isn’t already. Strictly due to usability.
They serve different purposes because Matrix is built more for communities and Signal for instant messaging. But if you’re deciding between any sort of encrypted messaging app, I strongly prefer DeltaChat, which is basically a WhatsApp clone in the way it looks and functions. It is not good for large communities because it doesn’t have “servers” and “channels” and all users have the same authority. Meaning anyone can invite and kick anyone else from any chat. But it beats Signal since you don’t need to give it any personal information.
It’s just an alternative DeltaChat client made by one of the Delta developers. I’m not sure why it exists instead of just upstreaming the things that are different. It runs on a different “instance” though, kind of like Lemmy. You can get it on FDroid or using Obtanium though. The F-Droid page shows a lot of differences and it’s supposed to have enhanced privacy over regular Delta.