The difference is bio-motor compatability. i.e what is your goal / activity/
Pullups have the upper body in closed chain & lower body in the open chain & pulldowns are the reverse, which do you need?
As it was stated before, pullups give greater fiber stimulation, overall strength and development. It is always easier to move a weight toward your body in a fixed postion than it is to move your body towards a fixed axis
both can be be beneficial, all depends on your needs