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Silverlight is primarily for text-based applications, whereas the XNA Framework is primarily used for games. Both of these frameworks are flexible and can be adapted so that your imagination is the only limit to what can be created.
Regardless of which framework you decide to work with, you will still have the benefits of the Windows Phone Application Platform. The toolset is free, a rich application programming interface (API) set is available across the entire platform, all applications are sandboxed, and finished products can utilize Marketplace distribution routes.
By the Way: Choosing Your Framework
To choose an application framework, you need to have a very good understanding of your application idea itself. To help, consider writing a paragraph about what you want your application to do. You can also note other applications with a similar style to what you envision. Remember, a little preparation goes a long way.