The XBAP-deployment model in WPF offers you a way to implement rich browser-based applications. Silverlight holds the same promise. So I was wondering: “What’s the difference? Why choose one above the other?”

According to the WPF documentation

“XAML browser applications (XBAPs) combines features of both Web applications and rich-client applications. Like Web applications, XBAPs can be published to a Web server and launched from Internet Explorer. Like rich-client applications, XBAPs can take advantage of the capabilities of WPF. Developing XBAPs is also similar to rich-client development.”

Silverlight is described (by Wikipedia) as

Microsoft Silverlight is a runtime for browser-based Rich Internet Applications, providing a subset of the animation, vector graphics, and video playback capabilities of Windows Presentation Foundation.

Security considerations.
This seems one of the major differences. XBAP-apps normally run as partially trusted apps in the internet zone and thus have very limited access to critical system data and resources. This means you’ll probably have to resort to Isolated Storage for your file IO. Alternatively you can write you app so that it demands full-trust permission from the user.
Silverlight-apps obviously run in the Internet zone and can also demand full-trust, so in terms of security, XBAP and Silverligt seem equivalent.

Cross-platform/cross-browser
XBAP-applications require a Windows-machine to run one; in fact, the user should also have the .NET 3.0 framework (or later) installed to run the apps. If the user has .NET 3.0 XBAPs can only be loaded in Internet Explorer. As of .NET 3.5 (see also yesterday’s post) FireFox can also be used.

Silverlight doesn’t require the .NET framework, but users have to install the Silverlight plugin for their OS and browser before they can run the application. This plugin has been released for Windows, OS/X (Apple) and Linux, and runs in a plethora of browsers. So, in terms of cross-platform/browser capabilities, Silverlight is a clear winner.