Slack vs Telegram

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
Telegram is a messaging app with a focus on speed and security. It’s super-fast, simple, secure and free. Telegram seamlessly syncs across all of your devices and can be used on desktops, tablets and phones alike. You can send an unlimited amount of messages, photos, videos and files of any type (.doc, .zip, .pdf, etc.). Telegram groups have up to 200 people and you can send broadcasts to up to 100 contacts at a time. Be sure to check our website for a list of Telegram apps for all platforms. Telegram alternatives for business are Slack, Hipchat, Microsoft Teams
Slack has better video than Telegram