Electric Brush versus Manual Brushes
There have been multiple studies comparing the effectiveness of manual brushes as opposed to electric brushes.

Although not all electric brushes are the same, these studies conclude that in general electric brushes are more efficient in controlling plaque than manual brushes. Theoretically, you can do a very good brushing with a regular hand brush, but the movements of an electric brush make the task easier and more effective.
Also, some electric brushes (Sonicare) produce sonic vibrations that are difficult to mimic with a hand brush. Other electric brushes like Oral-B have small heads that help you access hard-to-reach areas of your mouth. The heads of Oral-B brushes mimic the polishing cups used in the dental office and are very good at keeping the lower front teeth clean.
Our office recommends the Oral-B Triumph. It has clinical studies backing up its performance and it's the brush that Dr. Mohajir uses on a daily basis.