They're very similar in most respects. I found Mattermost much simpler on the operations side, and easier to write plugins for. We ended up choosing it and has generally been happy with it.

If you find this kind of telemetry suspect, you are probably better off looking at Matrix. It is a functioning open source project, as opposed to open source-as-entry-level. The functionality is mostly there, the drawback we found was high resource usage.

For friends-and-family type usage the ecosystem around Matrix, and the huge number of clients being worked on, will probably make it the better choice in the long run.