It is a good idea when buying any power supply to check if there is a fan profile available from the manufacturer, this will let you know fairly precisely how the fan will behave at idle and under load for the particular model concerned. Sometimes PSUs in the same model range use the same fan profile, sometimes they are different. In this case the 720w has a different fan profile from the 620w. Looking at the two profiles it seems to me that the one for the 620w is less quiet, with the minimum fan speed being higher than it is for the 720w and the 620w profile also runs the fan at higher speeds as load increases. So yes, the 620w would be noisier than the 720w.
From the Cooler Master website, this is the page for the 720w M2
and this is the one for the 620w
. The fan profile is found part-way down the page under 'More highlights'.