I think pull downs are the better exercise for bodybuilding purposes in most instances. Achieving good form on a pull down is relatively easy, yet rarely seen. Achieving good form on a pull up is a bigger challenge. It's easy for me to do pull down's and pull with my lats while keeping my chest up and properly retracting my scapulae. With pull ups, focusing on muscles is much more difficult -- I'm just trying to get up to the top of the bar in the first place.