I found a nice table which will show a comparison between Microsoft products and Flash/Flex:
| Features/Benefits | Silverlight | .NET | Flash/Flex |
| Rich 2D animation/graphics with audio and video | yes | yes | yes |
| Industry standard video codec | yes | yes | no |
| Scalable video format from HD to mobile | yes | yes | no |
| Hardware-assisted editing and encoding solutions | yes | yes | no |
| XML (XAML)-based presentation layer for SEO | yes | no | no |
| Choice of standards-based and high-performance languages | yes | yes | no |
| End-to-end server and application platform | yes | yes | no |
| Media server licensing (unlimited bandwidth) | $999 | $999 | $4500 |
| Content access protection (DRM) | yes | yes | no |
| Client side playlists for ad-insertion | yes | yes | yes |
| Robust video publishing tools and third-party ecosystem | yes | yes | no |
| High-performance, multi-core enabled client | yes | yes | no |
| Scalable full screen video up to HD (720p) | yes | yes | no |
| Native support for device-based video | yes | yes | no |
| Offline, document support | no | yes | no |
| Data Exchange with Web Server (Ajax, Web Service) | yes | yes | yes |
| Encrypted Content | no | yes | no |
| Binary or Text Format | text 1 | binary | binary |
| Easy Installation Support for Platform Requirements | yes | no | yes |
| Tools | yes (Beta/CTP) | yes | yes |
| 3D Support | no | yes | no |
| 3rd Party Controls | yes 2 | yes | yes |
| Client size | ~2MB | ~50MB | ~2MB |
| Supported operating systems | Windows/Mac
3 | Windows
4 | Windows/Mac
Linux
Mobile (light)
5 |
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. 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...