If they're worse are they equivalent?
The big difference is in vibes. Slack is unashamedly corporate and would love to sell you premium SSO integration. They could cut out (or offend) the free community tier, no sweat. On the other hand Discord is more gaming aligned, and would love to sell you some premium custom emojis. Offending the free tier users or getting rid of the free tier is a lot less compatible with their business model.