Michal Terepeta
Hope you don't mind if I add an opinion of a small/occasional contributor to the thread.
Personally, I would prefer a move to GitHub. Mostly due to familiarity and network effect (pretty much everyone is on GitHub).
But I would also consider a move to GitLab a big improvement over the current Phab-based setup. A git-based workflow would be great - I use arc/Phab too rarely to really invest in learning them better. (I just figured out the simplest way to use them that seems to work and I'm sticking to it :) I haven't actually used GitLab before, but it seems super easy to sign in using GitHub credentials and the interface seems quite familiar.
One thing that was already mentioned is the ticket handling and I just wanted to say "+1". I *really* dislike Trac - it's slow, unintuitive and every time I use it I need to spend a couple of minutes to find the guide to its own weird version of markdown... So a better place for tickets that's tightly integrated with code code hosting/review tools would be really cool! Which brings an interesting aspect of this discussion - if I had to choose between "GitHub for code hosting/review & Trac for tickets" vs "GitLab for everything", I'd prefer the latter.
Thanks for this data point, Michal! Cheers, - Ben