WPF applications are basically for windows client
applications. Silverlight and XBAP are designed for web applications. So if you
are targeting for only IE and Firefox then choose XBAP. Otherwise Silverlight
is the best option as it can be run in any browser. XBAPs also require .NET
framework 3.0 in user's machine. So if you are creating an application for your
company or intranet where each machine has the .NET framework, then go for
XABP.