With Adobe and Microsoft pouring millions into their next generation RIA platforms, which will dominate? Flex is obviously the current leader having been around for years before Silverlight was released, but Microsoft seems determined to make up ground with large investments in the platform and tools (the Expression suite.)
I am a fan of Flex, but I certainly prefer C# to ActionScript for coding. I prefer Adobe's design tools, but Expression Design and Expression Blend are impressive for v1 releases. I really like the way they integrate with VisualStudio 2008. I can create UIs using a designer friendly tool and then code behind them with a developer friendly tool. Flex Builder gives you a similar development scenario but the designer is not nearly as powerful as Expression.