Used correctly both work well.  For some children the electric tooth brush may motivate them to brush more often.  The whizzing sounds of an electric toothbrush and the tingle of the tufts swirling across teeth and gums often captivates people that own electric toothbrushes.  They are advantageous because they can cover more area faster.  Electric toothbrushes are recommended for people who have limited manual dexterity.