The general consensus to that question is quad. It obviously runs hotter and can't overclock as well but it's more future proof. Now if you're planning on upgrading in say 6 months then i'd say definetly just get the E6850. Most programs today don't utilize 4 cores so it will beat it due to the 600 mhz speed advantage.
OR.. You COULD overclock and get 2 in 1. Buy the Q6600 and oc it up to 3.0 ghz fairly easy on stock cooling.
I'm stuck in the same position. I want a E6850, but a Q6600 is very appealing too. Ill still have the computer in my sig though, so I'm very close to settling on the Q6600.