Which builds the biceps better -- the hammer curl or the biceps curl? Let's begin by recalling that the function of your biceps is not only elbow flexion but also supinating your wrists -- essentially turning your wrists so that your palms are facing up. You can actually see this happen when I supinate the biceps.
So by supinating on a dumbbell curl, we're actually able to train that function which we're not able to do as well with the hammer curl. You should still do hammer curls with a neutral grip if your goal is to train the brachialis. But if you want to build biceps, then the biceps curl is the exercise of choice.