from my experience... sound levels are hardly any different... sure you can concentrate sound in certain areas of the case... but you still need to put a 120mm on the 120mm radiator... or if you use a 240mm radiator, your using 2x120mm fans... some people even double up fans so 4 for a 240mm radiator...
not to mention to get good performance with low speed/low noise fans you'd probably want the expensive radiators... which thus blow the prices out of proportion anyways
and meh, I cant see a 120mm radiator giving temperatures much better then stock... if anything it might be worse since GPU's put out a buttload of heat