One thing I can't help but notice is that Discord and its API are pretty unreliable. My bot and users regularly encounter unavailability errors. But mainly my bot.
It makes me wish I could pay a monthly fee to get better service, but in reality I'm sharing the same freely-distributed resources as any spammy Discord server. For example, I've been in Discord servers incessantly spammed by its users counting to the number 1,000,000, messages scrolling by like access logs during a DDoS. And I'm sitting there chuckling, "my server is in the same bucket as these people?"
At least Slack lets you pay them for an uptime SLA. I don't think I'd use Discord for a serious venture.
That said, surely your niche plays a large role in which one you pick. I'm somewhat in the gaming niche and Discord was a no-brainer. Gamers/teens are already on a dozen other Discord servers. Meanwhile, all of the Slack groups I'm a part of are professional.