In reality, the choice usually isn’t that hard. If you already have a rich client application that was done with XAML and C# (WPF), and a subset of this functionality should be offered for Windows Store apps, the easiest option to do this is to choose XAML and C#. You can build up on your knowledge on developing with C# and XAML. And it’s also possible to share code between desktop apps with WPF and Windows Store apps by using the Portable Class Library. With this project type you can select the frameworks that should be supported, e.g. .NET 4.5, Windows Phone 8, and .NET for Windows Store apps:
Of course the design of the WPF app needs to be changed to fulfill the new Microsoft Design guidelines, and to integrate with the charms bar, put commands into the app bar, and more.
And of course, if you have a C++ desktop application, you can take the knowledge to write C++ apps for the Windows Store. XAML is now a great offer for C++ as well.