or yet another take....
it is said that your average air cell is 6 inches. given this, if the seeing is bad then a 4 inch aperture apo will show better detail than a 8 inch dob because... well its obvious, the 8" dob will take more than one of these aircells while the 4" will only need one. or so i am told....
given average to good seeing the 8" (all things equal quality wise) should blow the 4" out of the water giving much more detail even though because of the central obstruction the stars won't be as neat...
well this is my take on it anyhow. I dont know if its 100% right but it seems to work in theory.
