You can walk into almost any store and the tooth brush sections is over filled with many different choices. What is the best toothbrush? How do you know which one will be the best? It is simple it is the one you are going to use. There is no real different between an electric toothbrush and a manual as far as one being better then the other. It is all in what you prefer.
Manual tooth brushes are normally cheaper in cost then the electric ones. They are easier to travel with and cheaply replaceable. Electric toothbrushes vibrate which some people like the feel on their teeth and gums. Some have timers on them so you can make sure you are brushing at least the two minutes recommended time period. Kids tend to like the electric ones just for the fun of it which may make them brush more.
So try out the different types and see which ones you like and will be willing to use as recommended two times a day.