Well, as I mentioned before I only use 1000kbps settings, so there isn't as much difference as with lower rate heavily compressed videos, but I did notice that with ON2 you get smaller file sizes, sometimes almost 1/2 . Had one 4mb file exported using sorenson spark, and then using ON2 plug-in came out at only around 2.5mb.
( other times it's much closer )
It does seem that sorenson has better contrast and richer colors, so I guess it's still up to the individual user to decide what they prefer.
Another advantage of sorenson is , you don't need to export for flash 8 which is only at around 50%-60% penetration right now, older versions are in the 95% range.