If it's just a plain snaffle, there really isn't a difference between the eggbutt and the d ring. The full cheeks, however, give you a little more control because they're harder to pull through your horse's mouth.
As far as everyone who thinks us working cow horse people are using curbs - again - not quite. We ride with SHANKED bits. However the grand majority have broken mouth pieces which enable us to pick up and isolate parts of our horse for the outstanding moves you see.