@BShark so I checked out the discord server you posted, and it seems like the basic feature sets are pretty comparable, the notable exception in favor of discord being that it has more prominent built in audio in the room. Slack has a google hangout integration, and slack has announced they'll do their own video and audio, so it probably won't be long until it has a comparable presentation of audio in the chat to discord. Personally, I don't want an audio feed most of the time during games. I'd turn it on when Mario hits the shot, etc, but I don't know... I'm a homebody that watches basketball with my family and types fast. But, I also get the appeal.
I'm comfortable administering Slack and adding integrations / grooming it because that's what I use at work, so that's why I went with it. Plus its a multibillion dollar company; I think its safe to assume it will keep getting better, will have tons of maintained integrations, and will have a mass of people that will already be familiar with it. It has a nice mobile app, and a robust feature set.
So for now, I'd like to stick with Slack, unless there are some critical drawbacks that are addressed by another solution.