Even between the Q9550 and the Q6600 both at 3 GHz, the Q9550 would have the edge due the higher FSB while running at 3 GHz (353 x 8.5 vs 333 x 9) and the optimizations in the 45nm quads.
^^^Use google man....as already posted by vrd, AMD and Intel use different architecture, now if you need to know the difference in architecture then read the technical papers........
Definitely the Q9550. Apart from the lower power consumption, heat, higher overclockability, higher FSB and the larger cache, there's also some SSE4 and VT improvements in the 45nm quads which all give an edge over the Q6600 depending on the application.
I myself run VirtualBox for running multiple OS and I have successfully ran Ubuntu, Open SUSE and Windows Vista Ultimate full screen all at the same time on top of my Windows XP without any noticeable slow down.