It really comes down to the details. If the little things don't get on your nerves and you just care about putting messages in a IRC-esque interface that everyone else can see then there's no difference. Here's a really high level tldr: Teams is annoying in dozens of small minor ways that are collectively very frustrating.

For a more detailed take:

Slack is far from perfect but Slack is generally clean, quick, simple and compact. Slack feels like a purpose built tool for chat. Even when they add stuff like huddles it never feels like the priority is anything but the chat. When they roll out updates that make typing or formatting jankier I find they often get a fix out quickly.

Teams feels like it is doing a little bit of everything and is slower, wastes screen real estate, has poor UX all over and does a lot of things so-so to decent vs. one thing really well. Why is chat so different in a Teams/Chat vs a Teams/Teams/Channel? You can't even do the same markdown formatting in those places. And speaking of markdown, you can type a backtick around a string to monospace it like in slack but if it's at the end of your message it won't format it. You need a trailing space for it to kick in the formatter. Little details matter and a lack of attention to them as an organization makes bad UX and weird behavior leak into your software.