I use both and also BitBucket. Probably one reason to choose GitLab over GitHub is if you're offended by the Microsoft acquisition of GitHub. I'm not, so it's not a factor for me.
I primarily use GitHub for open-source stuff (here and here). It's free for that and it is the "go-to" place to find open-source projects.
I've been using BitBucket for my own private projects, either just personal or for clients. BitBucket is also free and they do provide the nice SourceTree Git client.
Lately I've started looking at GitLab and have put a few open-source projects on it (here) and I really like it's clean design and overall usability. I like that it is also free for private projects. I may start using this more instead of BitBucket.
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