I'll field the latter. I've used both Zulip and Slack in small team (5-20 people) settings and I much prefer Zulip.
In particular I like how the zulip UI is single-stream by default. Messages from different channels are interleaved, so you can read all new messages at a glance, but still well separated by visual elements including color - I don't find it confusing. And I can focus down to any particular channel , or reply to that channel, in one click.
It goes all-in on this "many streams, one view" model with the concept of topics - lightweight "subject headers" for individual sub-streams of a channel. This neatly avoids the "two conversations at once" problems that can occur in other systems occasionally.
Some more minor points are a wider range of markup available than slack (including syntax highlighting for code), and better support for short-lived private groups (ie. "a PM between Alice, Bob and Charlie")
As to what I think Slack does better: More one-click integrations (and likely other backend admin stuff that I don't see as a simple user). AFAIK Zulip doesn't have an irc gateway. Also when I last used it (over a year ago) Zulip's mobile client was quite poor.
I'm sure others can come up with more differences - to be honest I tend to treat slack as "slightly smarter irc" so I don't pay attention to all the bells and whistles.
Finally I should caveat that these same features I like are things people dislike about Zulip - it's more dissimilar to existing solutions and people will always have their preferred ways of doing things. I personally find Zulip amazing for me.
FWIW Zulip does have a beta-quality IRC gateway (bots/irc-mirror.py); anyone interested in working on improving that should check out this issue: https://github.com/zulip/zulip/issues/249
I especially like the concept of topics in zulip. It helps maintain different subjects in a given channel, which is way more readable imo. I'm a big fan.
In particular I like how the zulip UI is single-stream by default. Messages from different channels are interleaved, so you can read all new messages at a glance, but still well separated by visual elements including color - I don't find it confusing. And I can focus down to any particular channel , or reply to that channel, in one click.
It goes all-in on this "many streams, one view" model with the concept of topics - lightweight "subject headers" for individual sub-streams of a channel. This neatly avoids the "two conversations at once" problems that can occur in other systems occasionally.
Some more minor points are a wider range of markup available than slack (including syntax highlighting for code), and better support for short-lived private groups (ie. "a PM between Alice, Bob and Charlie")
As to what I think Slack does better: More one-click integrations (and likely other backend admin stuff that I don't see as a simple user). AFAIK Zulip doesn't have an irc gateway. Also when I last used it (over a year ago) Zulip's mobile client was quite poor.
I'm sure others can come up with more differences - to be honest I tend to treat slack as "slightly smarter irc" so I don't pay attention to all the bells and whistles.
Finally I should caveat that these same features I like are things people dislike about Zulip - it's more dissimilar to existing solutions and people will always have their preferred ways of doing things. I personally find Zulip amazing for me.