WinRT has so many choices for developers in term of language for Windows Store app development, that you have to really think which one is better (if not the best) among the all equals.
I have listed down some basic guideline which can help you decide between XAML (.Net developers Favourites) and HTML5 (the game changer)
- Learning Curve: There is a famous saying "A sword never kills anybody; it is a tool in the killer's hand", so if you have to make a choice, consider your team strength in terms of XAML vs. HTML5. If you are starting with a clean slate then in my opinion HTML & java script is quick to learn as compared to XAML, but this does not mean that you should ignore XAML, because if it benefits you in long run you might still want to spend that extra time in learning XAML.
- Use Case : If you know that you are going to deal with lot of HTML content and RSS feeds, then HTML5 should be the way to go, XAML webview has many issues and is not as efficient in handling HTML content. And you might want to consider XAML if you dealing with lot of animation and 3D rendering
- Portability: If you considering porting your application to other platforms outside Microsoft eco-system like iPhone and Android HTML5 will be your friend. However if you do not have any plans for migrating/porting this app outside Microsoft platform ( and not dealing too much with HTML content) XAML might be a better choice because of rich tool set support like Blend and VS.
- XAML has more controls out of box as compared to HTML5 (WinJS) controls, but this again depends on the need to project