If you ever want to release your project, I think it's better to stick with GDscript. Because C# has some: "Sorry, that feature is not supported yet for C#", the biggest one I know is the Android export thing. Unless you are sure that you will not meet any complication with C# for your project.
For heavy processing work, you might try it in C++ and even build features in the Engine itself with C++ and recompile it for absolute performance.
Not sure how much performance you need. But I believe Godot team is going to improve GDscript's performance according to the recent QnA. But I believe making GDscript more strongly-typed is higher on the priority list?