There is a 3rd alternative...
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Why choose Flock or Mattermost?
In terms of features, both Flock and Mattermost offer roles management, custom domain name, chat, file sharing, mentions, groups/channels, GIF and emoji, 1-1 video call, screen sharing, audio call, and "Do not disturb" mode. However, Mattermost offers more advanced features such as group video calls (up to 20 participants), threads, multiple workspaces, unlimited storage, task management, two-factor authentication, and the ability to import tasks. On the other hand, Flock offers notes-taking, participant permissions, and training resources, which Mattermost doesn't have.