Don't forget Silverlight is going to require a plug-in to use, and to my knowledge it has not been "natively" added to IE, let alone the rest of the browsers. So there could be tons of maintenance/support issues with that alone. PDF files are considered "ubiquitous" by now, but you still run into a user or two that doesn't have Adobe Reader on their computer and it often occurs at a bad time and then you're scurrying around to get an installer.
At a fundamental level, this is what has kept me from doing Silverlight for my web apps. I think the technology behind it is good, but considering nowadays you could get equal visibility/functionality with a nice Webforms/MVC/AJAX/jQuery combination (mix and match to your liking), I'd say stick with ASP.NET.
