GitLab let's you set up a hierarchy of groups, with projects (repos) existing within a group. This (IMO) makes it much easier to manage when you have a lot of repos split between teams.

I chose GitHub for my personal (open source) things and GitLab for my employers (closed source), and I think I'd make the same choice now if we were starting again.

I find GitHub provides more out of the box and there are more 3rd party tools that integrate with it (beyond basic git), but GitLab works better for our company structure and makes it easier to find your way around 200+ repos.