Silverlight offers cross-domain support for applications,Silverlight plugin is just a 1MB download and is supported in windows XP and
above.Silverlight also has a browser plugin but still needs to reach at pace with
flash.
supports asynchronous calls
Benefits of Ajax
Ajax is absed on open standards so widely accepted and supported.Bandwidth usage of web is less as specfic demands are accomplished.Works with Flash and FlexKey component of Web2.0 apps
benefits of regular web apps like low cost etc.supports asynchronous calls.
Disadvantages of Ajax
Ajax also uses javascript,CSS and XHTML but there is an issue of cross browser
compatibility that is what might work on one browser might not work on another
browser which makes the debugging difficult.
Benefits of Flash / Flex.
more reference show here
http://blogs.msdn.com/piotrp/archive/2008/06/23/instantiating-silverlight-html-vs-script.aspx
http://silverlight.net/getstarted/
http://faq.programmerworld.net/programming/silverlight.html