Since I'm being quoted here, I feel obliged to step in.
I stand by this comment:
"The main benefit of higher power PSUs is when the airflow in the PSU is deliberately set very low in order to minimize noise. This means the PSU components will run hotter. All other things being equal, a higher rated PSU will be a better choice in such an application because its parts are generally rated for higher current and heat than a lower rated model."
With most decent brands, the highest rated model in a series DOES
have higher rated parts than the lowest rated one. This means the higher power one will tolerate more heat longer than the lower power one. It does not always mean that the higher power one runs its fan at a lower for a given power output; that really depends on whether the PSU designers went to that level of detail to minimize noise.
Seasonic has mentioned that they use the higher heat headroom in their higher power models to shift the "turnover" or ramp-up point in the fan controller to a high temp on the higher power models to keep noise to a minimum up to a higher temp/power level on those models. They are the only ones who have mentioned such a strategy; it's also very difficult to veryfy without an absolute thermal-controlled test enviroment.
My original comment is most relevant for those who want to mod their PSUs with a slower fan. So if you take a PSU that normally runs its fan at 20 cfm at 40C, and replace it with one that gives only 10 cfm at the same temp, all other things being equal, a 500W model should withstand the higher heat considerably better than a 300W model.
But there's no clear evidence that in most PSU lines, the 500W model will run quieter than the 300W one -- at any temp. And as Jan Kivar mentions, with some higher power PSUs, a more powerful fan is used to accommodate the higher heat output potential at max power, which further negates the "quieter" potential of the bigger PSU.