I don't think this is a super useful comparison, because the two services have wildly different threat models. I think of Matrix as a secure replacement for Discord. Signal is about small group messaging. It's literally a replacement for the built-in texting app on your phone, and that's its intended userbase. Signal is what you use when you need to know, to the limit of best practices available to ordinary users, that your messages will be as private as they can be made to be. That's a goal that isn't compatible with many of the affordances people want for project discussion platforms and things like that.

If you pit Signal against Matrix and make the competition purely about security, Signal will win for the foreseeable future. But I think it makes much more sense to think about different sets of tradeoffs being more appropriate for different kinds of problems.