I just wrote a comprehensive article on which codec to use for screen captured content, and I also talk about VP6 vs Sorenson Spark Pro along with H.264 and WMV v9

If you guys are interested check it out

here is the link
Best codec for Screen capture content (benchmark)

I would really liek to know whay On2 thinks about it. The quality of ON2 is that great, but its good enough. definetly betetr than Sorenson Spark pro