I wanted to expand a bit on your point about the IDE and dev/designer workflow - I've been working with Silverlight for a year and half now, and I have to say the key to my success has been the tooling. On the dev side the ability to step through code in the debugger from client side to server side across a web service call is very helpful. We've hired designers with experience in the Adobe toolset and seen them become immediately productive in Blend (animating UIs, transitioning screens, hiding/showing elements, etc). Couple that with the fact that both Visual Studio and Blend can share the same source control system and you've got a great ecosystem for rapidly pulling together good looking web apps.
One other pro for Silverlight is the language independence. If you choose C# you also get LINQ, lambda expression and (soon) parallel foreach loops.
