Mattermost sports higher performance, uses Postgres, is a pure server side application not using uo too much resources on the client side and in general gives an overall enterprise grade impression.
Updates go smooth without a hassle, everything is logical, and the integration in the OS is absolutely stable. Apart from that, the underlying runtimes and code are mature, proven and stable.
The developers are maybe a bit more cautious regarding introducing new features, but they maintain a stable experience, not breaking the codebase in order to hastily implement new features, which are not yet ready for production. Documentation and debugging are fantastic, so running this in enterprise production environment is absolutely approved of.
We tried out a handful of communication tools including Slack, Microsoft Teams, and Google Hangouts Chat, before settling with Mattermost.
The customisability offered through your server's system console is unrivalled and in some ways overwhelming with the sheer amount of options that you're provided with.
All communication tools share 99% of their UI with each other, and Mattermost is no different, but that's not a bad thing. It also seems to have a less cluttered interface than the others we had tried, although I can't pinpoint the specific design choice that is the reason for this.
The fact that we can have control over all of our data (we're self-hosting it through AWS on a single EC2 instance) is also a great plus which none of the options that we looked into offered.
We chose RocketChat over other communications suites like Cliq or Slack mainly because we can self-host it on our own infrastructure. Since we have quite some projects going on which demand that we stay in touch with a lot of different stakeholders, pricing was an issue, too. With RocketChat, we have a huge set of features basically for free, RC offers apps for all major devices and systems and overall, we're very happy with it. The only downside is the limited amount of apps and integrations, but we can make due with what we have available.