About the servo motor:
To select the servo is actually selecting the servo's motor first. There are 3 types of motor used for model servo: iron cored motor, coreless motor, and brushless motor.
iron cored motor: long history, most highly used,cheapest. Tradition big DC motor is all made by this type of motor. Drawbacks: low efficiency, high inertia, big starting power.
A big improvement for tiny DC motor. High efficiency, low inertia and small start power.
Drawbacks: Still a brushed motor. There's a elec brush in the circuit to change the current direction. It's hard to get high reliability.
A revolutionary improvemt for DC motor. High efficiency, middle inertia, amd small starting power. No brush in the circuit any more, which increases the reliability to a great extent.
High prime cost but with low difficulty to produce. The cost mainly comes from the sophisticated elec chips. However, along with the descending price of the elec chips, the brushless motor shows its highly cost effective side.
So how to choose the servo with these types of motor? If you have no demand of performance and reliability, choose iron cored servo. If you have the demand of performance, but no consideration of reliability, choose domestic coreless servo. Its price is middle level and it has good performance. We suggest to select high torque servo with the satisfactory speed. If you demand high performance and reliability, choose brushless motor.