Which one should I use?
All students, staff and members of faculty can use Teams. Teams is a cloud service and also includes a powerful collection of collaboration tools where you have access to video meetings, shared areas, collaborative writing and more.
IT staff and teaching staff can use Mattermost. Students enrolled in classes where Mattermost is used, can also use Mattermost, of course. Other staff may also use Mattermost, please see the Mattermost web page for details.