You have already made two points very clear:

• This is a GUI application. So GUI is not just a nice add-on.
• You don't care whether it is coross platform.

Go with C#! When it comes to GUI application on Windows, even Java is not a choice any more (if it was). I believe you wants high quality GUI application. The look and feel is definitely the very basic and essential. If your GUI application even does not look like other GUI applications running on the same box, doesn't matter how rich its functions are, it is not a good GUI application.

I do Tk programming, but that's only when the targetted users are myself and several other IT users. If it is for my business users, C#. We have tried Java and Web, at the end we picked C#. (For GUI application, Java's development effort is higher, and it does not really look Windows-alike; The development effort for Web application is very very high, when the GUI interface is complex enough.)

The development effort for C# aplication is comparatively lower.