Matrix is slightly different to XMPP in this respect, as rooms are shared over all participating servers. So if the matrix.org homeserver went down (as it ocassionally does), all the users on other servers continue without impact. As such, it's more like email than XMPP.

Meanwhile we're also working on supporting decentralised accounts in Matrix, so users can pick which set of servers they can use as a homeserver. This then gives the best of both worlds: a simple thinclient protocol that anyone can implement, and knowledge of precisely which servers and devices host your data.

Basically, Ring and Matrix have totally different architectures, and which you use probably depends on what you are optimising for :)