I use both on a daily basis for my work and the various clients I work with:
Slack:
Pros:
You can create channels (public and private) based on interest, groups, sub-groups and history isn't lost.
Good one to one calls, new feature of huddles allows people to join / leave audio call as needed. Which works well for helpdesk / support groups as someone can always be on the huddle and you can join to just ask a question then drop off.
Good mobile app
Cons:
Channels become very unwieldly very quickly and currently I am a member of probably 50 channels and spend 10-20 mins a day just making sure I am ontop of everything
Private chats are limited to 8, then you need to move to a channel, all conversation is lost
Can't change between public and private channels, so some quite important channels were created private so new team members "just need to know" and get invited to them
Audio isn't as good as teams in large groups
Screen sharing is iffy, and definitely not good for large groups (5+) on an audio call
Once a private chat has started, you can't easily add or remove members you need to start a whole new chat losing history.
Teams:
Pros:
Tightly integrated with AD/Office so meeting invites in office include teams link with the click of a button
Video and audio conferencing that works with with larger groups like 30+ including sharing screen
You can add/remove people to chats and then allow them to see all history
Federation if the org allows outside your company to other O365 orgs is super easy, with audio / shared screen for the most part just works.
Cons:
The app is buggy as hell on Windows, Mac, iOS and Android. Be prepared to restart it at least once a day. Also when you change your AD password it can take 30 mins or longer for the password to sync, so if you were talking with people all of a sudden you get kicked out even though you changed your password ages ago.
It doesn't play nicely with VPNs and needs a restart when VPN is up
There isn't really channels per-se so it is just a individual chat between one or more participants.
If I had to chose my preferred tool it would be Slack, but the only thing that would be good is more control over the sprawl of channels... ugh.


