My architect’s view is that there are two main factors that affect a decision on this type of development platform.
a) How much on the leading edge do you want to be?
I see Silverlight trailing behind Flex in innovation, but only initially, with the gapusually shortening over time in Microsoft catch-up fashion.
b) What is your team’sdevelopment experience with Microsoft (or Adobe) web development platforms?
You need expertise in the complete platform in order to squeeze the most out of the features available. An experienced team needs to be ready to create innovating new techniques to handle the limitations of an immature platform that will be frustrating to an inexperienced team.
If the delivery and support model of your product/project is
a) To be delivered and supported within a 24 to 36 month timeframe
b) if it’s a specific tool that needs to be delivered and not a whole platform (and local Air type implementations are not needed )
…..then Silverlight could meet the needs as well as Flash.