My recommendation is Direct3D for game rendering, optionally with a XAML-based UI system.

Right now on Windows Phone neither Direct2D or DirectWrite are officially supported for third-party Windows Store apps, so Direct3D is preferable from a portability standpoint. Its true that D2D and DirectWrite are user-mode libraries written on top of D3D, and that people have gotten it to work in Windows Phone apps, but still current MS policy prevents these APIs from being called by apps distributed through the store. This may change in the future, but there's no knowing for sure. The details of Xbox One indie dev haven't been widely revealed, so we don't know whether Direct2D or DirectWrite will be available on that platform, or denied like on Windows Phone. If you only care about Windows 8/8.1, and not Xbox One or Phone, then this may not concern you.

XAML is a nice UI system in that its skinnable, extensible, and supports both mousing and touch. Right now XAML on Windows Store apps and XAML on Windows Phone are similar in concept and relatively similar in execution, but not in such a way that's directly portable. That said, the expected direction among the community is that Phone XAML will be replaced by the Win8/8.1 version, although there's no official statement from MS to back this up.

As an alternative, Cinder is also a nice library that's been getting a lot of press lately, and there's a port to Windows that supports Windows 8/8.1 store apps for both normative and and RT versions of the OS. It's a great, easy-to-use library that cuts through all the boring DX boilerplate you'd normally have to write.