Okay, I am totally biased toward Silverlight; I could tell you a million reasons why you should be building Silverlight apps, all of which you can take with a grain of salt due to my bias as part of the Silverlight developer content team. However, one of my former coworkers from the WPF developer content team, Jim Galasyn, who was a bit of a Silverlight skeptic, has written a compelling post as to why you consider Silverlight if you are not already developing apps with it. Jim takes a look at building the same control for COM/ATL vs. Silverlight vs. Android vs. Phone 7. I think you will find his conclusions interesting. Check out his post here.