When to pick which

If your team is engineering-heavy and would have stayed on Slack for the code blocks, integrations, and threading, Mattermost is the closer feel — its UI clones Slack's left-rail / channel grid almost line-for-line, the Slack import CLI ports your history losslessly, and the Postgres backend is friendlier to ops teams who already run Postgres for everything. Mattermost also wins on lightness: a $5 VPS comfortably hosts ~50 daily-active users, half what Rocket.Chat needs because Mongo is the heavier process. The trade is licensing — Mattermost's Enterprise tier (LDAP groups, compliance export, retention policies) is Source-Available, not OSS. Pick Rocket.Chat if you're running a public community or a customer-support org: the built-in livechat widget, omnichannel inbox, and looser MIT license make it the natural choice when 'users are not employees.' Avoid Rocket.Chat for tight engineering chat — its default UI feels heavier and the Mongo footprint adds a backup story your dev team likely doesn't want to own.