Brandon points out one of the main advantages that Silverlight has is the "Speed to Production" - getting the app delivered, out to production quickly. The advantage is better responsiveness and agility for the business. Unfortunately this usually equates to less time for proper testing, and especially performance testing.

It's also interesting how he points out performance comparison at the Ux layer - which I think could be described as the "perceived performance" for the entire application system. In Enterprise RIA or mashed-up application users might not perceive on-par performance from either Flash or Silverlight, depending on the backend systems. In a big RIA, you have multiple points of exposure to latency risk introduced in the data services calls behind the application - so even if the UI is responsive to the user, the data retrieval might be slow. Check out James Ward's blog on "Flex data loading benchmarks" - showing the combination of AMF and BlazeDS, which is proving to be a very scalable and responsive combination.