Microsoft makes jumping onto Teams pretty compelling, by essentially bundling it with Office 365. The integration is well done. The cost is low (once you're in Office-land). However, Slack's usability still completely stomps Teams
- Workspaces: as a consultant, I can't be logged into all of my clients' Teams at the same time. Teams client won't allow more than one Office 365 login at a time, so unless they have a guest user policy, I have to have an anonymous browser window open for each additional Teams-using client. This is an epic PITA. Slack makes this so easy.
- Channels/conversations: Slack's channels are quick to set up, intuitive, and easy to use. Teams' channel equivalent, called 'teams' (brilliant) sucks. They tend to quickly turn into abandoned spaces. Notifications are crap. The message threading in them is awful and confusing. Teams needs to make its replies flat. Half the time you think you're replying in a sub-thread, but you end up replying to the whole team instead.
- Teams notifications. So far, nobody I know has been able to figure out how to enable notifications for a Teams team (ugh), unless you're @-mentioned. So you may miss out on important conversations unless you remember to visually check your Teams client.