Discord feels less polished to me. I find push notifications way less reliable, and often if I do get a notification the app won't refresh for a few minutes to show the message. I think there's no offline handling at all for notifications, so if you're offline when a message it sent you're never notified about it. Search is basically unusable if you want context, they only let you see a few messages after your search term. Also they don't have outgoing webhooks which make bots more awkward.

It is nice that it's free and unlimited of course, it's a no brainer compared to free Slack, but for work I'd prefer to use Slack.