Gitter works okay for me but I do expect it will never have the tooling and capabilities of Slack. On the other hand, Gitter will probably better integrated with the the project and open source culture. I'm personally preferential to Slack as this is my only Gitter engagement and I have 7 Slack teams. But I'll happily follow the project to whichever service is selected.
One thing I can say for sure- the mobile app support for both Gitter and Slack means I won't be offline for weeks or months at a time as I was with IRC.
I don't like having this awful separation of organization profiles in slack. This is the main reason I think it doesn't fit open-source community requirements. Still, there's a crutch for slack to allow open registration for everyone, but it requires setting up yet another service (heroku instance).
Yep, in most ways it is better. But the problem is that we can’t use slack here as:
everybody uses gitter
slack can’t be used this way
Which I, as I said before, is strange. It is not like slack is some super-duper advanced thing, IMO it would be pretty easy to add most of that functionality to gitter. On the other way, the isolated nature of slack teams makes it impossible to use for public projects.