Ask HN: HTML5 or Silverlight for a new Startup?

I am about to create a start-up offering an online service to consumers. Consumers that are not as tech savvy as HN users. We are talking about average mom and pop type users.

I want a rich user experience, something like a desktop quality application but on the Web. I have been considering Silverlight because it is already available and allows a rich application to be constructed. But I am thinking that the extra steps of getting the user to install Silverlight might be a issue with adoption.

So the alternative is HTML5 and using the Canvas. But is that really a good idea with the current state of adoption in browsers and standardization being at an early stage?

Which direction would be the best medium to long term choice here?