Personally, I don't like either. Neither of them do threads in a clean and accessible way.
Teams has a learning curve that's too steep for a comms tool, and some highly questionable UI design aspects ('read more'? really?)
Slack I find clumsy, poor performing, and also has some poor UI aspect (chats that vanish from the sidebar being my pet hate).
As for the alternatives...
Google Hangouts worked well enough, but doesn't do code blocks so is borderline useless. 20 seat limit on conf calls can be a pain.
Apple iMessages / Facetime is Apple only.
Jabber/XMPP/IRC would be too hard to bring on board most places, for less technical folk. No screensharing.
Email isn't collaborative enough and also can't share screen.
Skype has taken a very strange direction and is mostly unfit for anything other than one-to-one video.
Facebook Messenger? haha. No screensharing anyway.
It feels like there's room for a more polished and sophisticated chat/collab/video/voice/screenshare tool.