Microsoft has done a great job with Silverlight 4, but many people don't know what the primary differences are, specifically as it relates to search engine optimization (SEO). The purpose of this blog post is to illustrate the primary differences between Silverlight and Flash (from an SEO perspective).
The bottom line is that Silverlight is more SEO Friendly than Adobe Flash
Silverlight 3 is more SEO friendly becasue it permits deep linking URLs to point to places within the application or website. To make things even more exciting for search engine crawlers, an ASP add-on mirrors dynamic content into HTML for easy indexing. Microsoft has included an XML presentation layer in Silverlight to address a major problem with Flash. With Flash, you need to use the SWFObject to provide copy to search bots. Google chooses to overlook XML that is application-specific, as would be the case with Silverlight XML.
Silverlight treats text as a separate entity on a web server, and subsequently applications are fully searchable and indexed by search engines. Though Flash has traditionally not been very SEO friendly, Adobe is working in conjunction with Google and Yahoo, have had some success in making Flash more friendly to search engines. But for now, Silverlight is the clearn winner.