Thanks for initialising this one Bryce,
In github's favor is that it holds the greater mindshare. Where we to go with it we'd potentially tap into a larger community of developers, which potentially could translate into a greater level of new participation in the project. github also seems like it's received a greater amount of polish and has some feature advantages (github's CI was seen as a huge pro last time we looked).
gitlab is an up-and-comer and is actively acquiring analogous features to github. A big pro for us is that gitlab is FOSS whereas github is free but proprietary. With gitlab we'd also hold the option of self-hosting, which might not matter or might be a huge advantage, it's hard to say.
One advantage to GitLab is a contact on the inside. If we need to, we can ask questions directly.
I've worked with Github, Gitlab, and a few others in my contracting work, as well as private Gitorious and self-hosted gitlab. I'm currently in favour of GitLab as the most in keeping with our Free Software ideals, as capable as GitHub and the possibilities of lending a hand with improvements to things like the issues tracker in the future to make it work better for us.
GitHub is decent, but it's fashionable but with some downsides. At least they recently fixed their "nothing has a license" bug.
I should add that both GitHub and GitLab have weak project control. I don't believe there's an advantage to either and misappropriation of the inkscape brand is highly likely on both. This is probably a personal bug bear as they both go for the "repository is king" model.
Best Regards, Martin Owens