This is a tough one. Silverlight is better than flash for video at the moment. It's actually the better technology on that front. But, a lot of people don't have it installed which makes it a barrier to entry for some. There are a lot of people who won't install it and would miss out on the video.

My stance on all of this stuff is that you use technological solutions to meet ministry needs. The technology is always just a tool and never the ends. Based on that I would not use it until Silverlight adoption is up. Your goal is to spread a message and you want to keep the barrier to entry as low to hearing that as possible.

I would prefer to watch video in silverlight. But, I know my mom would not install it so I would never put a video up in silverlight for her to watch. It's about the people and the mass church audiences aren't ready for it.

Hope this helps... just my 2 cents.