I definitely prefer Slack. Here are a few reasons off the top of my head.
1. Many users have issues where the unmute button is unresponsive. Typically the workaround is to turn off incoming video feeds which isn't exactly ideal. This is a major turnoff.
2. Difficult to stay involved in multiple channels as described by another comment. I find it tedious to find what's happened if I've been away for a while.
3. Slack has bots that do various helpful things including leaving channels you don't use, asking you for your daily standup feedback if you don't have an in person meeting, referencing external content, etc. I assume Teams must have something similar but I've never seen them used which likely indicates its a lot easier to do with Slack.
4. Nicer UI as you allude to yourself
5. Slack works with several video chat options including their own (based on AWS tools I believe?) and Zoom. I actually prefer Slack video chat over Zoom or Teams for pair programming as its so easy to annotate the screen without clicking into a bunch of menus or being the meeting owner or something. You literally just start drawing on the screen and your coworkers see it
6. Excellent mobile app which is quite important for a chat client
7. Ability to run a small chatroom for free which AFAIK Microsoft doesn't offer (its included with other Microsoft products that were paid for usually)
8. Not tied in to only Microsoft products