The Q9550 is much faster with its 12MB of cache, will overclock better being its 45nm, Runs cooler and just all around way better processor.
Now is it worth spending the premium for either of these processors? No
You would be much better off getting an I5 750 or a Phenom II x4 955 both are a little faster then the Q9550 and will be alot cheaper. Unless you are upgrading a current socket 775 system and can get it for under $200 otherwise move on to a new platform.
I got them off EBAY for a song spent less than $200 for both of them