We should be seeing the EO stepping Q9550's soon. They are suppose to be better than the C1's, or we want them to be.
Alot of motherboards hit the wall @1600mhz FSB so keep that in mind if you pair up to a Q9550. It would be nice to have unlocked multipliers on the "Q" series.
Both will be able to hit 3.4ghz easy assuming you have proper cooling. If you don't have good cooling the Q9550 will probably go further with less voltage; however with the price difference you can easily buy a Q6600 plus good cooling compared to Q9550 plus stock cooling.
q6600, because it leaves you with a substantial budget to get the best cooling. The worst case is you would be left with the budget for a swiftech H20 compact setup, OR the best heatsink and bigass fan out there.
id say the q6600 because as state cheaper and you can get an awsome aftermarket cooler for the same price as the q9550. or if you really want to get the best processor out of your money go with the q9550 and a artic cooling freezer pro 7 ive heard really good things about them and its what i plan on getting for my q6600