Recently had to decide between Silverlight vs Flash for a project.
* 2.0 would support IronRuby
* Already have development tools
Flash & Flex pros:
* Installed everywhere.
* Tons of resources
* Lots of preexisting stuff I can look at.
Since I already had the MS tools and Flex builder would have cost me some extra change, I was leaning to Silverlight -- until I found out it doesn't support Webcam / Audio!
When deciding on which major technology to use, always determine that all prospective contenders support your required features!