I would say the American Bulldog is the superior protector and working dog over all. The Bullmastiff has been watered down and truly breed for the show ring. When it comes to American Bulldogs, you will find several strains. I would first say research the varying strains of American Bulldogs, performance ( scott ), Johnson, Painter, Screaming Eagle, Hines, and of the Hybrid Bulldogs which consist of crossing several strains together. The Bullmastiff at one point was a great working dog, they are currently plagued with hip and elbow problems and watered down temperments. The biggest key to owning any working dog is to Control ( ob ) and excercise, and proper socialization. You will have a big part in the way the dog intercacts with people.