GitLab is better in some ways, worse in others. One thing GitLab is much better at is supporting a rebase strategies, making for a much nicer history. GitHub still doesn't know how to show the difference between the original version and a force-pushed one when reviewing, but that just works on GitLab.

Another good thing about GitLab is that all the CI code is right there, not in a bunch of separate repos. You can reproduce it locally pretty easily.

The main thing GitLab is worse at is refreshing the UI when the underlying state changes, without having to refresh. They're getting better at it, but to be fair GitHub also has plenty of issues with this.