Ok, it's all about the cfm, the resulting noise for a given level of cfm, and then the rpm that's needed to achieve that cfm. Read that again, that is key.
The end result you want is higher airflow. Period. Now, to get a higher airflow, you need a bigger fan and/or higher rpm's. But with higher rpm's usually comes higher noise. So, it's a question of balancing the airflow with the noise. Some people want minimum noise, some want maximum airflow.
Maximum airflow = Silverstone FM121 (110 cfm, it's a beast)
Minimum noise = Noctua 120mm @ 800rpm
Balance of airflow and noise = Noctua 120mm @ 1200 rpm. These put some very good air and are absolutely silent. Better than S-Flex, better than Nexus. But for some people they want even more airflow...which is fine, but the fans won't be quiet anymore.
So, what do you want:
-high airflow and don't care about noise? Silverstone
-low noise and don't care much about airflow? Noctua, Nexus
-low noise and good airflow? Noctua
Lastly, if you want better control over airflow vs noise, then get a fan controller. You can adjust the voltage/rpm's to achieve what you consider to be a balance. That level is different for everyone, depending on what matters more to them: noise or airflow. So you just pick the range that works for you.
BUT, if you want quiet and airflow, you don't need a fan controller for the Noctua 120mm @ 1200 rpm fans.....they're already dead silent so no need to adjust them down to make quiet.......you won't even hear the whoosh of air at their top speed