Flash vs. Silverlight: a useful technical comparison here at Smashingmagazine.  And the winner by category:

Animation – Silverlight
File size – Flash
Scripting – Silverlight
Video/Audio – Silverlight
Sound processing – Flash
Accessibility – Flash
Platform compatibility – Flash
Text representation/SEO – Silverlight
Supported image formats – Flash
Socket programming – Flash
Webcam support – Flash
Deployment – Flash
Windows application – Flash
Media streaming – Silverlight

Other Adobe vs. Microsoft face-offs:

Illustrator vs. Expression Design – Summary:  Expression Design is not a head-to-head competitor to Illustrator as a standalone graphics editor, just a handy XAML-friendly tool to use with Expression Blend.

Coming Soon:
Dreamweaver vs. Expression Web
Bridge vs. Expression Media
Adobe Media Encoder vs. Expression Encoder