I would pick Flash for a number of reasons. It has a lot better x-platform support for it's IDEs. This is important because if you are creating anything at all

I would pick Flash for a number of reasons. It has a lot better x-platform support for it's IDEs. This is important because if you are creating anything at all rich you are going to have designer and coders working on your stuff. Constraining them to a single platform reduces the amount of people who would be willing to work on it. Flash also now has Flex which allows you to code and compile. Depending what you are doing this is great to have designers and developers on the same project. Finally people are used to building stuff using the Flash IDE. While Silverlight might be the best thing since sliced bread I'd rather use a technology I know I can just find a contractor for in about 10 minutes.