For my data intensive web application (heavy forms & complex reports), from performance standpoint, which is better ... a browser plug-in based RIA (say SilverLight) or Java Script based RIA (say ExtJS)?
At this moment, we can avoid the discussion of plug-in availability, etc. My only focus is performance. Reasoning will be appreciated.
Hard to say because it depends on so many variables. But you might want to check out Bubblemark. On that particular benchmark, Silverlight 3.0 (with bitmap caching enabled) does well, but a lot depends on the browser you're using. Javascript with Chrome comes pretty close to Silverlight's rendering performance. See also http://www.youtube.com/watch?v=gqjidzjkMM0.
If you use Silverlight and can access your WCF backend using Net.TCP, you can get pretty good data access performance as well. I would expect it to be somewhat faster than JavaScript (since it doesn't have the HTTP overhead), but I don't know how it would compare to accessing FMS via RTMP.