Basecamp vs Slack

Basecamp tackles project management with a focus on communication and collaboration. Making to-do lists and adding to-do items literally just takes seconds. Basecamp is optimized to make the things you do most often really fast and really easy. Basecamp mobile is especially made for popular mobile devices like iPhone and Android. Basecamp free alternatives are Trello and Asana. If you are looking for open-source self-hosted Basecamp alternatives pay attention to RedMine or ProjectPier.
Slack brings all your communication together in one place. It's real-time messaging, archiving and search for modern teams. Create open channels for the projects, groups and topics that the whole team shares. Slack searches whole conversations, not just individual messages, so you can find what you’re looking for no matter who said what or when they said it. Slack free open-source (self-hosted) alternatives are: Mattermost, Rocket.Chat, Zulip
Basecamp has better video than Slack

2015 - Enterprise messaging service Slack introduced User Groups to keep up with Basecamp

Slack is looking to enhance the experience of how larger organizations function through the introduction of User Groups. The new feature allows notifications to be specifically sent to separate departments. Whether it’s getting the message out to a company’s engineers, customer service teams or marketing teams, user groups allows entire groups to be alerted about issues in a moments notice. Initiatives like this fit into Slack’s larger focus of allowing the platform to operate more effectively for larger organizations. In turn, this feature will only be enabled for Slack teams on paid plans.