dumbbell curls isolate one arm from the other - with a barbell, you're lifting the weight with both arms and your stronger arm can compensate for the weaker one; with dumbbell curls, that is not the case, as each arm works independently - also, with dumbbell curls, you can change the position of your hands (i.e. standard curls, concentration curls, hammer curls, etc.) and target the heads of the bicep in different ways. The most effective regimens include both dumbbell and barbell curls, for those reasons.