would you recommend Snikket server (or Prosody) for 1:1, group calls and screen sharing?

Answering this first so it doesn't get buried down. Screen sharing wouldn't be supported by xmpp since its just messaging, but I believe Jitsi has that feature. But for the rest, snikket and conversations (for android) I would recommend, yes.

When I decided to try XMPP, I had to do a lot of research to decide which applications I should use for the server and client.

Whatever is the first answer you get from a web search should be fine. Most sources recommend conversations for client, but all the other recommendations you'll see are good too. For server, the easiest to setup is snikket, but all the other and up to date implementations should work okay, although they might need some configuration if you want all the modern messaging features.

If we told two people to use these two software independently, they would start using Matrix much more faster than XMPP.

Why do you think so? Let's assume a user who doesn't self host. XMPP clients are far more stable and error free, whereas matrix has random issues every now and then, especially with encryption and public groups.

XMPP clients are a lot more customizable and come in different models. Matrix has only one client that works well (and some forks of it that look roughly the same). I'd say that's a win for XMPP for new users.

Now let's say it's a self hosting user. I don't need to say much here, matrix is notorious for self hosting issues, and being a massive resource hog. XMPP, you have snikket, which works out of the box without issues and can be hosted on a raspberry pi even.

I may be biased here, so I urge you to tell me, in what way would a new user adopt matrix faster? I can tell you one. Matrix has corporate funding and has managed to advertise better. That's their only win.