Here is the breakdown according to Robert Evans, Microsoft Technical Lead for Windows Store App Labs (Source):
- 59% XAML/.NET (C# and Visual Basic)
- 5% XAML/C++
During my inspection of apps, there seems to be even a higher percentage of XAML/.NET apps; however, I trust the submission numbers. Just the ones I tend to use are generally written in XAML/.NET.
You can inspect your installed apps, too. You can look behind the Windows 8 curtain.
Whatever you pick, enjoy it.
Update: These percentages are based on over 4000 apps that have come through the app labs. These are mostly consistent with the numbers that I have found through limited inspection. Here is how: http://www.pursuitofgreatdesign.com/2013/02/a-look-behind-curtain-of-windows-8.html