Studies show electric toothbrushes, especially oscillating types, remove slightly more plaque and are easier to use well, which helps people with braces, arthritis, or a tendency to brush too hard. Built-in timers also encourage the full two minutes.
That said, a manual brush used correctly cleans teeth perfectly well and costs far less. The best brush is the one you use properly twice a day; technique and consistency matter more than the technology in your hand.