Not sure if this helps, but we were evaluating Mattermost and are still evaluating Zulip at the moment. Our users pretty fast had a tendency towards Zulip, while I've had a slight preference for Mattermost from a ops view.

We only have/had a short eval phase and our use case is a small raceengineering team with 40 people at the racetrack, independent of internet access. So a very particualar use case, and our needs might not cover you needs.

Zulip pro

* Threading model (can't value that high enough)

* All messages overview, no matter what stream they were written in

* Zulip community at chat.zulip.org with direct contact to the devs. Tim Abbott the lead dev helped me out in a issue with a user login (haven't used the Mattermost support so can't compare)

Zulip con

* Most ops work has to be done by a python script manage.py, API has room to improve

Mattermost pro

* API

* haven't measured but the UI felt smoother, faster

Mattermost con

* you have to pay for Enterprise features

Both have great docs and a docker deployment