Doesnt that basically answer your question....
If overclocking, I would say the Q6600 would win. For 3.6 u need a FSB of 400 (3600/9=400).
For 3.6 on the Q9300 requires an FSB of 480 which pushes the limit on many motherboards.
The new Penryns also have slightly better performance than the C2's at the same speed and SSE4 instructions, but no one knows how well they will perform with software that supports it.