See, right away someone has to talk bad about anything other than Kennedy, when they don't know any better on how they function.
Get the facts first, but this is typical with Kennedy LP owners, only buy/use what I've bought.
All 3 are good quality LP's
Your over exaggerating on how AirDog and FASS functions.
There is NOT a constant flow to the ENGINE, only to the liftpump, what's not used by the engines demand (lead foot) returns back to the tank via return line from the liftpump.
The ONLY time you'll have air in the return line is when a customer falls for buying the larger LP's 150, 165 with a faster flow (larger 3,000rpm motor), not needed unless your planning on having 600 plus HP.
I went from a FP-100 (first model) to a 4G165 (warranty) having this Niagara falls return with air bubbles you mentioned then back to a 4G100 (new model 2,000 rpm motor) nothing but fuel returning with no air in the fuel viewing inside the fuel fill neck.
I would prefer to have fuel readily available to the engine with the push of my lead foot.
Oh, did I mention you have a lifetime warranty on AirDog and FASS motors.