Alex Zambelli has provided a simple rule of thumb in his Media Processing Workflow slidedeck:

–H.264 decoding is typically more CPU intensive than VC-1 decoding for the same resolution and frame rate
–In Silverlight 3: H.264 decoding requires about 15–25% more CPU time than VC-1 decoding with similar content properties

Good rule of thumb (for now):
–For HD video, use VC-1 to reach largest audience
–For SD video and smaller, use H.264 if quality gains are noticeable

This “(for now)” part is supposedly a hint that SL5 will/should/could feature GPU video acceleration. Now that would be really cool. With cross-platform video playback being one of the biggest (if not the biggest) forces to win Silverlight installation base, I wonder why this feature hasn’t got it into SL4 scope…