I have to use both for various client projects and here are my thoughts. While GitHub is very popular and now has the financial backing of Microsoft, GitLab has an extremely powerful and seamless CI/CD compared to GitLHub's. While GitHub is making great progress in this area GitLab still leads in DevOps IMHO. Also, you can set up your own private GitLab server locally for your group. Overall, it comes down to which interface you are used to. Both are great products and excel in their own ways.