I haven’t logged into it yet so I don’t really know. You can connect with all the other nostr accounts there. I assume its just a nostr dm service, like messenger for facebook. But Sx is a messaging app of its own. And 0xchat will be private. Not like the public ones we have in nostr clients like Amethyst. And I've also noticed that you can zap one another from inside the messaging app which you can not do from Sx. The sat transaction will be private.