Q6600 has a FSB of 1066, and a 9x multiplier...Q9450 has a FSB of 1333, and a 8x multiplier...Q9450 is about 10% faster clock per clock, that means a q9450 @ 3.0GHZ = 3.3GHz Q6600...
Majority of ppl on here recommend the q6600 because it has a higher multiplier and thus easier to overclock, however recently its overclockability has gone down as ppl have trouble getting past 3.2GHz...
Q9450 is also 45nm while Q6600 is 65nm...Q9450 = newer and better but $100 dollars more