Thank you !
I would prefer XAML over HTML5 as it provides flexibility, performance and again that is in accordance with my experience. Jerry Nixon from Microsoft has a few good reasons supporting this - blog.jerrynixon.com/2012/05/windows-8-why-i-choose-xaml-metro-over.html
I personally prefer XAML over HTML.. And that's true with most .NET developers. Microsoft internal dev teams spent a long creating and optimising html / winjs. While it cant match xaml and c# performance isn't so bad.
I'd advice to do what you are comfortable with.
Hi, I prefer HTML5/JS the support and community is bigger, hence, the information on web is more complete