The c/s area of a 120mm fan is more than twice that of an 80mm fan. So to obtain the same massflow through an 80mm fan, you need to spin it much faster, hence its much louder (and higher pitched in noise) and much shorter lifespan.
Ah, I see.
Area of 80mm fan=3.14*(.4*.4) or ~.5 square mm
Area of 120mm fan=3.14*(.6*.6) or ~ 1.1 square mm
So a 120mm fan has slightly more than twice the area or an 80mm fan. So, two 80mm fans will have almost the same area as one 120mm fan.
Now that I search for fans, I see that most 80mm fans move 29-34 CFM and 120mm fans between 60-79 CFM. Would two 80mm fans use more electricity than one 120mm fan?
I guess I need to buy some good 120mm fans.