I think they serve different (though related or overlapping) purposes.
Briar started (IIRC) as a Bluetooth-only comm tool, and they've done a great job expanding what it can do (think it does Tor now?). Briar is not battery friendly, and the devs will tell you so. I don't consider it a daily driver, but rather for specific circumstances. I keep it around just-in-case.
SimpleX is more of a daily driver since it's a more conventional IP networking app, though it's a little battery hungry too.
My main concern wit Briar is that it would be of not much use without a smartphone (I meant the internet-less features in particular). I would not trust sensitive things to a smartphone. I wonder if soemthing like that could be doable with an Android VM or Waydroid with a laptop's bluetooth...
Maybe.
Check out Android Subsystem for Windows. It's essentially an Android VM though you don't have a launcher/home screen. You just see the apps in your task bar like any windows app. I run it on an older laptop, it's a touch slow but works well enough.
Yea, it's a different tool. And it's still early days.
I don't use SimpleX as a daily driver, yet. But it has a lot potential. Just glad to see another tool out there, and the devs seem really earnest (I worked with them a year ago while testing the app).
God how I despise SMS, and I can't get anyone off it, even if other options are easier to use than SMS, much more robust, faster, more flexible, etc.
There are a couple messaging apps that are self-hostable (like I believe SimpleX is). Litewire is one. At some point I plan on hosting one myself, and preconfigure accounts for friends/family to make it even easier for them. Maybe that will get them on board.
It's more or less truly anonymous chat. Like you meet someone on the street and need to chat with them, but don't want to give them any personally identifiable info.
It's really cool in concert, but good luck getting anyone to use it. Signal is good enough if you're paranoid. TBH Telegram secret chats are just as good for sensitive stuff and way easier to get folks to use.