C# is a native choice for C++ developer moving to .NET. It is really easy to learn C#, and you can find that Windows Forms is much better UI framework, than MFC. Windows Forms is actually one more native Windows API wrapper, based on GDI, it is designed better than MFC, and much easier to use.
However, Microsoft considers Windows Forms as old technology, and promotes WPF, which is based on DirectX and not GDI. DirectX is native API for Windows Vista and later. This is first difficult decision - whether to use Windows Forms or WPF. Learning WPF is not so easy, as Windows Forms, but possibly moving directly to WPF is smart choice. For example, Visual Studio 2010 UI is written in WPF.