I like the single arm version of the dumbbell for a few reasons:
1)Placing the resistance on the opposite side of the working leg requires a bit more core stabilization.
2)I feel that more force is transferred to the working leg since it has to counteract the resistance on the opposite side.
3)When working with two arms, lack of grip endurance (which many people do not have) may cause the set to terminate earlier than desired for the second leg. For example, you perform 20 reps with the left leg holding dumbbells in each arm. But when working the right leg, your grip is pre-fatigued from working the left leg causing you may be only to get 16 or so reps. Is that confusing? I hope not.