You don't have to choose. It's git. It's decentralized. You can have your cake and eat it. Have most activity on github, but a dormant gitlab as a backup in the (improbable) case it reveals unsuitable. Github is not facebook, you can get easily all your data back, in a open format.
Except for issue/project tracking. And all ACL and organisation configuration. And CI hooks and so on. Of course GitLab lets you make a clone of this information, but that's a hack.
GitHub has REST and GraphQL APIs you can use to export that data. It would be great if there were more standards for issue tracking/code reviews that worked better across vendors, but GitHub is neither the best nor the worst in this regard.
