IMO Slack is a much easier way to communicate and share files with the community.
Slack and lisk.chat looks the same to me.
Slack is more useful when you're following multiple slack channels. While RocketChat+ does support that feature too, lisk is the only altcoin that uses it.
I also like the fact that chat.lisk.io helps boosts traffic to lisk.io, I just wish Devs can confirm that they're not collecting IP addresses and matching them to ICO investors !
and not to forget lisk.chat data is not owned by third party (slack). good choice max