I'm a big fan of dumbells for various reasons. Because of the additional stabilizing involved, they promote balance not only of the different sides of the body, but of the strength ratios between the large muscles targetted and the smaller helper muscles and joint tissues involved. Practically speaking, that means less injury prone. Also, dumbells tend to show higher EMG activity on the targetted muscles in studies. Finally, dumbell training strength increases have been shown to have the greatest transferrence to the actual upper body movements involved in the more common sports.