Another question whose responses can be debated for time eternity. From my experience, I would say both languages have equally mature toolkits for creating user interfaces.
Hence for any project it only depends on what would be your preference. Generally, developing a UI using .NET is easier due to the fantastic RAD support provided by Visual Studio which unfortunately does not provide that level of sophistication for C++ based projects.
Having said that keep in mind that user interfaces exist on all operating systems; not just Windows. Hence you should focus on the basic tenets of GUI programming and treat the language only as a means of putting your thoughts into execution.
Hence if you really wanna do UI programming across all platforms; then C++ is the best bet ( of course these days you have Mono but not sure if they have a QT based toolkit you are looking at).
Quite a mouthful - but to make it short:
It is all about your choice and learning. If you wanna learn C# and use it go ahead. But if you are comfortable with C++, you can still use it and there is nothing that can be done in C# but cannot be done using C++.
