Signal is great for one on one conversations or small groups. Matrix is better served for communities as federation offers censorship resitance, but they collect some metadata that Signal doesn't. They do have encrypted DMs, but it's less user friendly than Signal and if your partner is not tech savvy, I would go with Signal.

Signal has 2 points of concern:

• Phone number requirement
• Centralized server (one point of failure)

I use both for different groups of people. E.g. My Matrix account is publically listed on my Lemmy account, but my Signal one is only given to people I trust to some level.

Go with signal, it's the right tool for your job. Matrix has its merits, but is less intuitive, less streamlined, and less polished then signal. For many use case these wouldn't matter, but for you they do.

Signal is so easy, my grandparents are on it. They can't attach a file on an email, but they can use signal.

I use Matrix (Element client on iOS) with the extended family and it works well. Most of them are not tech-savvy. I self-host my own Matrix instance because I wanted to have control on where personal pictures were hosted.

We should all be using Signal, all of the time. It’s accessible to those not technically inclined, and I feel like that’s a requirement now if a personal-use technology wants to get off the ground.

Edit: this is not to detract from matrix, which seems like a great technology for privacy

If you want to turn a normie off secure messaging for life, give him matrix. The UX is completely horrid.

Get Signal.

I have both and the synchronising of matrix keeps ages.

Matrix IMO is more like a discord alternative. For private messaged I would use (and I do use) Signal. Its very easy to setup too and seems like the most reliable instant messenger.

You don't need same app on both for phone to use matrix, there are multiple apps that can communicate over matrix, some may not be available on both iOS and Android.

But if you were to use matrix, I'd recommend using Element which just happens to be available on iOS and Android.

I use Matrix with friends and family, we all moved from signal.

I made accounts for my family members on my homeserver and told them to sign in, so they didn't have to register (slightly easier on boarding). They all figured it out with minimal hand-holding.

For my friends and coworkers, I just tell them "make an account on X instance, its a good one", and they all figured it out.

anything that gives you E2EE with keys/certs you control.

You would need to host your own or join a matrix server. The benefit of Matrix is you can tie in puppets to things like iMessage, Discord, FB Messenger if you host it yourself. Another option is Beeper (Matrix + all the puppets for other messaging apps) but you still have to sign up and get an invite from them to join for now.

If privacy is the ultimate goal, I think Signal is a bit better.

That said, Signal is doing a bunch of user-unfriendly stuff that turns me off a bit. For example, they had a great SMS integration on Android that they're now killing for no obvious reason. And more problematic- on iOS there is NO backup/restore functionality. None. So if you lose your phone, all your chat history is gone. It doesn't backup to iCloud or anywhere else. The ONLY backup or transfer option is if you get a new phone you can transfer data from old to new.
Android has a full backup/restore function that backs everything up to an encrypted file. No idea why they don't do the same on iOS.

Matrix is also better for multiple device access. On Signal, you can connect additional devices (laptop etc) but they are always subservient to the main device. Conversation history doesn't transfer from the main device to addon devices, although conversations stay in sync on both devices from the point you add the device forward. But if you get a new phone for example, that's a new parent device, so your desktop convo history gets wiped.
Matrix on the other hand, no device is primary and conversation history is stored (encrypted) on the server. So backup your crypto key and you can access everything from any device (including a web browser).

For Matrix- Element is the one to use most of the time...

I ditched signal after they reversed course and dropped support for SMS in the app for bullshit reasons. Who knows what other brain dead move they will try to pull next?