I have personally dealt with the 2 pumps you are referring to. The sedra 5000 created WAY more bubbles than the 3500. They both use the same venturi the 5000 just seems to be able to take better advantage of its design.
take a look at the skimmer that Jamie has in this thread
https://www.maast.org/forums/showthread.php?t=41163
That is a stock sedra 5000 and the body on that skimmer is 8". The bubbles are THICK.