I don't know as much about the larger paint guns, but in the airbrush world the gravity feeds do not require as much air pressure and therefore their spray pattern is more precise and leaves much less paint in the air and more on what you are painting.

The siphon feed require more air pressure since the paint is fed to the nossel from a vaccum created by the flow of air. It takes more air pressure and makes for a higher rate of overspray.

I would assume that would also apply to the the larger paint guns as well...

Dave Brisco