Several scientific studies have shown electric toothbrushes to be more effective at removing plaque (bacteria) from teeth and reducing gingivitis (bleeding gums). Having less plaque on your teeth will help reduce the risk of developing tooth decay.
Manual brushes are cheap to buy, easily portable and can be effective at cleaning your teeth if used correctly. You are more likely to be able to scrub and potentially over brush your teeth/gums with a manual brush. Sometimes the size of a manual brush can be too big to get to the harder to clean areas of your mouth.
Electric toothbrushes are more expensive to buy and need to be recharged on a regular basis to be effective. They will remove more plaque from your teeth if used as directed and will help people with limited manual dexterity and those who tend to rush brushing their teeth. Timers and pressure sensors will help do a better job and prevent over brushing. The smaller head will make it easier to clean the harder to reach areas. Your teeth will feel cleaner having used an electric toothbrush.