How is servo work? The principle is very easy. You can see there are three wires connected to the servo: power supply, GND and the control. The power is normally from 4V to 6V. The Control wire controls the angel of the servo.
Control wire will transfer the pulse to the servo board and command the servo motor to move. You can see the different wave length will result of different servo angle.
The different of analog servo and digital servo is the cycle time of pulse so digital servo is much more fast. Therefore accuracy of digital servo is higher. Then why we still use analog servo? Since digital servo have to send out more pulse so it is more power consume and the life of digital servo is shorter.
Then how we choose the servo? The quality of servo is the combination of many factors, such as the servo board, gear, motor and case. These will affect the accuracy, life and temperature of the servo. If you ask me for recommendation, Futaba is good at making good servos and the price is acceptable. In additional, you need to know what RC model the servo need to use for. If you are playing a 1/8 RC Off-road vehicle, the deviation is a few mm, if you use digital servo to correct a 0.1mm deviation, it is a waste right?
Buy a good servo from RC FEVER.