I found a nice table which will show a comparison between Microsoft products and Flash/Flex:
Insert table here...
1 maybe will be changed later; currently you could GZIP the content to reduce size of XAML code 2 using wpfeControl.createFromXaml(xamlstring); 3 Windows Vista and Windows XP Service Pack 2, Microsoft Internet Explorer 6, Windows Internet Explorer 7, Firefox 1.5.0.8 and 2.0.x; Apple Mac OS X, Firefox 1.5.0.8 and 2.0.x, Apple Safari 2.0.4 4 .NET 2.0: Windows 2000 Service Pack 3, Windows XP Service Pack 2, Windows Vista; .NET 3.0: Windows Vista and Windows XP Service Pack 2; Internet Explorer 6, Windows Internet Explorer 7. 5 Flash Player 8: Windows Vista and Windows XP; Internet Explorer 6 and Windows Internet Explorer 7; Windows 2000 with Internet Explorer 5.x; Firefox 1.x, Mozilla 1.x, Netscape 7.x or later, AOL 9, Opera 7.11 or later; Mac OS X, Internet Explorer 5.2, Mozilla 1.x, Firefox 1.x, Safari 1.x or later.
Will Ballard is writing in a comment about client performance. Well, this is very hard to compare because it will vary on some points. If you have a look at .NET (Click-Once or XAML Browser Application) you will have the same hardware support as installing the software directly on the client: 2D/3D graphics card, hard disk to store data and documents (compared with Cookies or Ajax server-side store),...
The performance of Silverlight and Flash will be nearly the same, I cannot see any difference. While Silverlight is using XAML as description language in a non-compressed format Flash will be faster for initial start. For Silverlight you could compress the XAML files with IIS compression enabled. There is a great Web site which simulates a Windows Vista: http://www.windowsvista.si/main.htm [1]. I think there you can get a feeling how the performance could be.
If something is wrong or missing on this table, please leave a message and I will add it. Maybe there are other platforms missing, please give me a short hint...