With the advent of Gitlab actions/workflows, it's hard to not choose Github anymore. I say that with all love for Gitlab, as it's been my personal tool of choice for a long time because of it's inbuilt CI/CD solutions. Github is just all around more adopted by the community so you'll always find more support; and if you go with enterprise you will get 50k build minutes a month as well as a ton of extra tools that will definitely help a startup out from the get-go. That being said, it's priced at $21 per user, per month so if you cannot afford that, I say go with Github.