There have been multiple studies comparing the effectiveness of manual brushes and electric brushes. Both brushes are effective in removing food particles, plaque, tartar and bacteria. It is important to aim to brush at least 3 times a day. If you brush after each meal and snacks that would be even better. Brushing along does not remove all the plaque and bacteria, especially in between your teeth and underneath the gums. Flossing is equally important as brushing, to remove the bacteria from in between your teeth. Professional scaling (scaling) is important to reach underneath the gums, which is extremely hard to reach at home, to remove the plaque, tartar and bacteria from the periodontal pockets. For adults, it is recommended to have scaling every three to four months and for children, studies show every six months is effective.
It is also important to spend at least two minutes to brush effectively. Brushing from the gum towards the teeth surfaces is best to avoid injuring the gums and causing gum recession. Remember to brush all the surfaces of your teeth including the biting surfaces. Most people do not spend enough time brushing the teeth at the very back and this is where caries often develop in adults. Also, it is important to brush your tongue since it can trap bacteria and cause bad breath.
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, studies shows that people are more likely to spend more time brushing with an electric brush since it is easier and takes less effort than a manual brush.
Also, some electric brushes (Sonicare) produce sonic vibrations that are difficult to mimic with a hand brush! Other electric brushes like Oral-B and Rotadent have small heads that help you access hard-to-reach areas of your mouth, such as the very back teeth. This aspect is more important when you are talking about someone with orthodontic braces or a history of gum disease. There are many types of electric brushes available, and ultimately it comes down to personal preference.