Why Discord ?
I can give some insight here.
In short I chose Discord at the time because it was a more "community-focused" platform and easier to set up for web-based communities.
In particular, what led me to Discord was: - the invite links, easily shareable Slack was still invite-only or with custom dev to do an invite page Discord just allowed me to post links wherever and people could join without even creating an account, which is great for one-off questions - the Roles for easily tagging people The lack of roles in slack really turns me off. In SFXD we have "normal" roles where you're grouped together in that role for brownie points, but we also have a rather sizable amount of "taggable" roles which allow you to ask a question and then tag the people that requested these roles so they can help. It's a nice way of limiting who gets pinged as you need to request it. - the concept of chat channels which are all "open" unless you don't have access to them It just makes channels more active by default. In Slack you need to explore the channels, join whatever. In Discord all channels you have permission to see are visible, which enourages more discussion.
I won't lie, the addition of the game store a yea or so back hurt us as far as IT blacklists go. I am still hoping discord releases something like Discord Prowith no gamestore that we could use or something.
But in the meantime as a community-focused hub Discord was just better, even if it's less prevalent - and despite its downsides I still think it was the right choice for us.
As far as federating Slack & Discord we'de need some sort of webhook that leads one into the other, I mean why not but that seems very dev heavy for not a lot of gain.