My gaming groups use Matrix, mainly for its stronger ecosystem and better long-term outlook. Despite developing slowly and not yet doing everything we want, Matrix is consistently improving and growing to serve more and more use cases. We're willing to tolerate some inconveniences for now, in exchange for having the contact networks we build today continue to grow for decades to come. We use Mumble for voice chat, because it's great, but might switch to MatrixRTC in the future (when Element Call leaves beta and becomes available in more Matrix clients).

I recently wrote up a few tips for Discord users considering Matrix.

If chat for a small gaming group was all I needed, I might choose XMPP. It's arguably easier to administer than Matrix once you learn about all the XEPs required for comparable features (ease of admin is relevant to me because I self-host) and I would be able to guide a small group in client choices and setup. But I have found XMPP's ecosystem to be a poor fit for large and diverse contact networks.