If you want to program in C++, that's fine. If you need to think about speed and memory management, C++ is the way to go. Of course, it's more complicated, and you will have to deal with compilers and IDEs. With regards to C#, it's a typed language that's a bit more forgiving. Though, you may not be able to get all of the features of the Godot Engine through C# (I can't remember if C# support has been fully implemented in 3.0).
Now if you just want to use the editor, with all of its features, go with GDscript.