If you don't plan on overclocking, the Q6600 is better.
If you do plan on overclocking, then the E6750 is better than the E6850.
For futureproofing, the Q6600 is "probably" better, but you can just upgrade to Yorkfield later on so I suggest buying the E6750 and overclock to at least 3.6Ghz which would be very easy with a good cooler.