They probably picked C# because most people who dislike GDScript, do it due to its dynamic typing. Plus, since Godot is an open-source project, C# is a great choice: it's pretty popular, resulting in a bigger userbase, which will eventually help in its development.
Now, personal experience: GDScript is a bit slow compared to C#. I've made an AStar class but whenever I called the function to find paths, I could notice stutterings, which are pretty annoying. Said thing doesn't happen when I do the same in C# engines. I'm also not a big fan of how identation works, but this is just me. I recall having this and this issue some time ago I deleted an entire project because it got me mad as hell. :#
