Depends on what you're going to use it for...
Stock - Q9300 is a little faster.
Overclocked - Q6600 is easier to run faster due to higher multiplier and lower stock fsb.
For video (specially HD) editing- no contest due to the SSE4.1 in the Q9300.
For maximum gaming fps, skip these two and get the E8X00 and use the money you saved to buy a good GPU.
My recommendation- get the Q9450 (double the cache of Q9300), a good MB and memory that can handle high FSB then use your imagination.