If you can figure out how to bootstrap it, Discourse will provide your community with more long-term value as it becomes a valuable, organized store of community information new members or potential new members can browse to learn about the community.
Slack is great for realtime interaction and a short term dopamine hit, but is also a black hole for information. This is especially true when you're using it for a community and not paying to have access to your old messages.
Yeah, basically I have a bunch of people who signed up to my programming courses and I want to give them a place to hang out, ask questions and interact with each other.
I've used Slack with other organizations and it really is a black hole. Not just for the 10,000 history limitation, but it's really hard to find and read previous conversations.
Slack is great for realtime interaction and a short term dopamine hit, but is also a black hole for information. This is especially true when you're using it for a community and not paying to have access to your old messages.