Just a small correction, the E8500 is not a quad core but a dual core processor. As such the Q6600 will probably be faster then it in rendering. But the E8500 will be faster in other parts of poser as those depend more on the 'GHz' value.

Personally i would get the Q6600 and overclock it to 3.16 GHz to get the best of both worlds, but that's only if you don't mind overclocking of course