Air Dog or FASS
I've ran both set ups and to be honest their both the same, except for price, size, and noise. I don't know about the new FASS pumps but they tend to be fairly loud. On mine you could here the pump over the engine if you were standing by the truck. Airdogs are insanely quiet, you can barely hear it outside of the cab with the engine off. And like Uncle Bubba said unless you really have the need for the air seperator, get the mitusa, its belt driven and is reliable.
Just like the rest of them has said you can't go wrong with either...it does make me laugh when people talk about the noise of the pumps when...most of them have straighted pipe trucks and 2 speakers in their back seat





