Yes, yes, I know that there's not technically any difference here; Matrix and ActivityPub are both protocols, efforts like 3rd Room are about building apps on a common protocol. I get that, please don't comment to explain it to me.
But in the way that Matrix and ActivityPub/Mastodon talk about unifying apps, there's a difference. Matrix talks about having one chat app that works everywhere because you'll have a ton of bridges. ActivityPub talks about having one media app that works everywhere because all of your apps will be built on ActivityPub or will talk ActivityPub. Matrix's "one app to rule them all" is based on trying to make itself compatible with everything. ActivityPub proponents want other apps to make themselves compatible with ActivityPub.
My instinct is that Matrix's approach is more practical, but it's hard to argue with ActivityPub's success. There's a lot of architecture decisions with ActivityPub (and Mastodon as well) that I don't really like? But the overall protocol seems to have worked a lot better than I thought it would. And even though I prefer Matrix's approach a lot of the time, as a regular user I'm a lot more likely to find myself actually using ActivityPub[0]. I use Matrix because I want it to succeed but I mostly just have it installed and do some family chats on it -- there aren't really any communities I'm following. I use ActivityPub because that's where everybody I follow went.
And it makes me wonder sometimes if my instincts on the "right" way to build a community are incorrect. Matrix has seen a lot of success in specific areas (government adoption for example), and it's made a lot more progress on solving problems that ActivityPub has kind of glossed over -- but it hardly ever even comes up in conversations inside of actual communities. ActivityPub does.
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[0]: Also yes, I know that they're not competing with each other and both can win; I'm contrasting them, not trying to turn it into a contest.