I'm debating whether to use Silverlight 2.0 vs ASP.NET MVC for a web application. The web application will be a subscription free service marketing all age groups. It's important the source is highly testable, but also with the Web 2.0 movement a graphical web application is important as well for competitive reasons.

I'm assuming silverlight is better than the ajax helpers/MVC graphically, but foundation-wise testing is better/easier with MVC. Possibly an MVP pattern with Silverlight could increase the testability of the source.

Could anyone elaborate on the pros/cons of each technology and recommend one or the other based on the above?

(addition 9/22/08) In regards to allowing search engines to index the site, using either technology it will utilize a backend database whereas a lot of the content will be dynamically generated. Based on some of the comments, when we talk of the searchable content would the home page of the application if written in silverlight be searchable? Would I be able to get the site to appear in a google search?