Matrix is far closer to being "the email for instant messaging" -- Signal-like double ratchet encryption by default for private rooms, federated in a similar way to email servers (but much simpler to manage as an admin), much nicer group chat UX, simpler identity key verification (it has Signal-like QR codes but it also has emojicode verification which is nice if you're not in the same place when doing verification), has an open specification, multiple client implementations, a couple of still-in-progress alternative server implementations, it even has bridges (which have pretty decent UX) to other social networks like IRC, Slack, Discord, and so on.

Signal is far from an open standard. I use it and am quite happy using it, but with the knowledge that Moxie has publicly said many times that he does not believe in federation or third party clients (and will implement client blocking as well as using trademark suits if the problem gets big enough). And it's fine to have that opinion, I just disagree with it and you shouldn't sell Signal as something the lead developer has said they're explicitly against.

For folks saying (read: hoping) that Signal may become federated in the future -- I think it's a much safer bet to just use Matrix since it's already federated and has at least equivalent privacy guarantees (or even larger if you self-host your own server) and hope that Signal will peer with them in the future (which I personally think is about as likely as Signal becoming federated itself -- namely, it's very unlikely either way). But you do you.