Chest flys with cables are better than dumbbells because you get a greater peak contraction on the top of the movement than dumbbells. You get a greater range of motion than you would with dumbbells on a cable machine during fly exercises. Also no matter how much weight you stack, it doesn’t interfere with hand spacing. Its also a safer exercise since you are closer to the ground with a ball on your back. I like to use a ball vs a bench because i am activating my core and leg muscles as well.