Lee, my experience is that suction fed brushes require higher air pressures and greater paint volume to achieve paint flow (have to have paint in the jar + siphon tube + airbrush at all times), plus I always tended to fill the jar with more paint than I needed. With gravity fed brushes you only drop in a few drops of paint and since there's no need for suction you can use lower air pressures. When painting large surfaces you still use probably close to the same amount of paint in the end, but on small details or multi-color applications you only use the amount needed, no extra required just to get the paint flowing. I find gravity fed brushes are also easier to clean since you don't have to mess with jars or siphon tubes. Which also means I can change colors faster.
Thats also one reason I like Vallejo Model Air paint so much. When I don't need to mix a color I can load the brush with the dropper bottle. No muss, no fuss.
I'm not the best at explaining things, so I hope that made sense :P