C# and C++/CLI dilemma

C++ is good for cases where you don't want your users to be required to install .NET framework.

C# and especially WPF is 10 times easier and faster for creating graphical user interfaces. Also, since it is managed code, you don't have to worry about memory leaks and that means faster coding. I haven't seen no performance issues with C# and I see no advantage in writing mixed code.

If you are ok with your clients installing .NET framework, C# is the way to go, definitely.