This isn't an argument. Merely pointing out the pros/cons.
Is the flow from the 20 smaller then the 30? Yes... His concern was flow not adding purgen. Im going to make a broad statement, most people Dont use purgen.
Switching the impeller is a neat idea but why... He doesn't have a spare impeller laying around. If you have to buy the filter who wants to buy a new impeller also?
I have a 20 & 30 and there is no noise diff in mine. My ac110 is the loudest...
flow seems to be his biggest concern. So I recommend the ac20. It is good to note that you need a ac30 if u want purgen.