I put all of my new open source stuff on GitLab - not because it's better per se, but because I feel like GitHub has a bit of a cultural monopoly and that an active competitor will push the entire community to a higher standard.
GitLab does have the advantage of an excellent, container-based CI... but there's nothing stopping you from using GitHub to host your repositories and GitLab to run your CI if that's what you choose.
Sadly I walked away from GitLab because gitlab.com seems to go down unreasonably often and I don't feel like hosting my own instance of it for the small projects I'm working on or sharing. GitHub has been perfect in this regard.
I'd go with GitHub, your potential users/contributors have a higher probability of having an account on GitHub which makes it easy for contributions. Moreover, there are a hundred free CI/CD services which work with GitHub and not GitLab.
You wouldn't know how many times they've stuffed the database, would you? As far as I know they don't have the same commitment to transparency for this kind of thing as GitLab has. We only know what they tell us.
As qw mentioned, we know of at least one time this happened to Github.
Doesn't really matter.
But I'd pick Github.
Github's familiar to more Devs. It's hard to find people using Gitlab but haven't used Github but the vice versa is fewer than few.
It would depend on your goals... If you just want to fit with everybody else, use github. If you don't mind the dual licensing model of gitlab, go with it. If you really care about open source, either use notabug[1], savannah[2], or self host your own. Personally, my preference is with notabug. I only use github extremely rarely.
Why not host it on GitLab and set up push mirroring to GitHub? You get the advantages from the extra features that GitLab offers and the extra visibility from GitHub.
Having a mirror on github is a common practice, but unfortunately, while you can turn off the issue tracker, github does not support turning off pull requests. So, you either need to include a prominent note that pull requests will be ignored and closed, or alternatively, have a workflow for handling github pull requests via whatever site you do use.
GitLab does have the advantage of an excellent, container-based CI... but there's nothing stopping you from using GitHub to host your repositories and GitLab to run your CI if that's what you choose.