[erszcz]

DISCLAIMER: I'm a MongooseIM dev.

Prosody doesn't scale horizontally / cluster [1]. It's a piece of cake to deploy it, but it's simply a different niche - you won't have a highly-available system serving hundreds of thousands of users with Prosody.

MongooseIM is a fork of ejabberd. The biggest benefit now (due to ejabberd catching up with some others) is an open-source huge XMPP compliance and integration test harness (ejabberd's open-source test suite is a joke and it's not known whether any closed-source suite exists). We pay more attention to failure scenarios in distributed setups (handle node downs, clean up transient data). Refactored code / cleaner internal interfaces improve iteration speed on customizations if you're interested in extending the system.

[1]: When I researched Prosody on this topic I found out that some clustering support was in development, but as far as I could tell it was far from usable. Honestly, I'd be grateful if somebody could shed more light on this.