At work for some reason we use both Slack and Teams because part of the company likes one and another part of the company likes the other, so i actually get to have simultaneous experience with both.
I use both on Linux via Firefox (their tabs are pinned side by side). I prefer Teams because:
a) Slack constantly forgets my credentials and i need to go through a whole dance of logging in, proving i'm human by clicking bicycles, having it email some code. Pretty much every day.
b) Slack, for some reason i cannot fathom, randomly switches me to "do not disturb" mode (or something like that) even though i'm right there. Not even away: it switches to a mode where i do not receive notifications when someone messages me. Fortunately it still does the beepy sound and i do keep the tabs visible on my screen all the time so i actually do notice messages, but i had at least a couple of questions from others why i am in that mode.
c) Slack does not support audio calls. Not sure why, but it doesn't work in Firefox. Teams does work just fine. Fortunately all work meetings are done via Teams and all people on Stack are also on Teams so if someone wants an audio call we use Teams, but still, a negative for Slack.
The only issue i had with Teams is that its text input can get confused when typing `backticks` and it seems to dislike using emojis at any place except the end of the text. Also Slack has some (old) meme emojis, though if that is a good thing or not depends on your taste :-P.
