Marko Beleuca

If you need just the source control feature, there aren't much difference. Azure DevOps has slight advantage since it has TFVC besides git. But, if you need some other features, like CI/CD, ticket/task management, etc. Well, that is completely another thing. For example, it is very easy to integrate BitBucket with JIRA. You could even combine BitBucket source control with Azure DevOps pipelines for easy CI/CD.

Go with the ASP.NET Core. It is a very mature technology now and there are tons of documentation, tutorials and support you can find online. Also ASP.NET Core Web API plays quite well with the React. It is easy to implement the entire back-end in .NET Core (APIs, authentication, database access layer...) and if you need any third party package, I'm pretty sure you will find and implement in a form of a NuGet package. Who knows, maybe one day you'll need to create a mobile app and with a fully functional Web API, it would be more-less easy task to build a mobile app on top of it.