So you have the client, you talk to the server, but in a decentralized system what does your server do? It talks to other servers! Wikipedia:Inter-server aka S2S is all about Server-to-Server communication protocols.
Interserver communication aka S2S
The most natural of the three is PSYC obviously. Since psyced thinks in PSYC packets internally (even if just in abstract data structures), it only needs to render them into PSYC format and cast them into outer PSYCspace.
Although the syntax remains the same, there is quite a difference in behaviours and functionality of S2S and Client-to-Server Jabber Protocols. XMPP S2S has some serious scalability issues described on the Jabber page.
psyced isn't very fast at parsing XML and generally dealing with XMPP isn't such a lightweight operation as doing interserver PSYC.
This is the newest freshest addition: psyced has the ability to connect to an IRC network disguised as an IRC server, and gateway an entire IRC network into federationland. This isn't a traditional way to do IRC S2S which in fact only a native ircd can do properly, and psyced will probably run into performance problems trying to keep up with the speed of an ircd and the size of a popular IRC network. psyced wasn't designed to serve large communities alone, so the natural way to make this work is to have several psyceds work as gateways into the IRC network in parallel.
We have been using this for BRASnet and it worked fine, but it needs custom patches for each flavour of ircd.