Depends on what you need... (Score 1)

II've in my experience learned that sometimes standards and compatibility aren't as important considering the audience. My clients are the ones paying for their RIA and the ones using it. If I had a choice, it's between HTML supported by AJAX or Flash. It's been around longer and has a huge community. If my client requires something that must be done in Silverlight (whatever that may be) I just don't work for them.