They have completely different architectures and philosophies. Matrix is a decentralised conversation database - it's all about synchronising communication his

They have completely different architectures and philosophies. Matrix is a decentralised conversation database - it's all about synchronising communication history between the different silos. It has a single monolithic spec, for better or worse, so you know that if your client and server supports the Matrix v0.1.0 spec, they will work with all the features available for the particular use case (e.g. mobile voip). XMPP instead is federated messaging and presence (and a whole bunch of stuff on top). It's passing stanzas between servers rather than synchronising historical stanzas between servers. The specs are highly modular and extensible and built out of XEPs. It's just a totally different way of approaching the problem of open communications. You could make the comparison that XMPP is a bit like SMTP, and Matrix is a bit like NNTP (Usenet). They both fulfil the same end goal - letting folks communicate openly over the internet. But they have totally different designs and there's room on the 'net for both. After all, you can just bridge XMPP to Matrix and everyone wins! :)