- 3D graphics are sexier;
- Runs as a desktop app and in the browser;
- Apps run online or offline (and run faster).
- Silverlight will make a very Rich Internet App (RIA). BUT you still can't print without a major code hack, and it has basic HTTP binding (no full security).
- If you have existing WinForm apps, WPF may be the better choice (it's a logical extension of your existing platform).
- If you have most of your stuff in ASP.Net, then start with Silverlight (it has a small footprint, produces great cross-platform apps, and is easy to deploy).
Before you blindly walk down the path, figure out what 'features' your end users want, and then figure out which product supports those features. Here are two MSDN blogs that compare the two technologies: Jennifer Marsman (a Microsoft Developer-Evangelist) summarized the differences (although the post is a year old); The Swiss MSDN Team updates it for Silverlight 3.