Brushed Servo Motors Most low cost / standard servos (analog or digital) use what is called a 3 pole electric motor. This is a standard 3-pole wire wound DC motor – the most common type of DC motor in existence. One step up from the 3 pole is the 5 pole servo motor. As you can imagine, two more wire windings will give a 5 pole motor quicker acceleration and more torque on start up.
You know by now the faster the servo ramps up to speed and the more torque it produces; the better it is for applications. Well, improving the electric motor itself will produce more speed and torque too; coupled with digital technology, the resultant speed and torque are indeed impressive.
Coreless Servo Motors A standard 3-pole wire wound servo motor uses a steel core with wires wound around the core, this core is then surrounded by permanent magnets. As you can imagine, the core and all that wire weighs a fair bit. When voltage is applied to turn the motor, it has to first overcome this weight to get things turning – it is slow to accelerate (due to inertial mass). Once up to speed, it also continues to turn for a while when the voltage is removed – it is slow to decelerate or reverse direction (again due to higher inertial mass).
In a Coreless design, the heavy steel core is eliminated by using a light weight wire mesh that spins around the outside of the magnets. This design is much lighter resulting in quicker acceleration and deceleration. The result is smoother operation, more available torque, and faster response time. I personally feel coreless motors offer the best performance - even over brushless (at least from the stand point of fastest acceleration and least amount of deadband).
Brushless Servo Motors The latest advancement is to use a small brushless motor in the servo. This is the exact same principle and has the same advantages as larger brushless motors that are used in electric RC helis, planes, and cars. There are no brushes that add drag or can wear out.
Brushless motors are more efficient, provide more power, and have a much longer life expectancy over coreless & brushed. They are however a fair amount more expensive and they have higher inertial mass than coreless.
The coreless/brushless debate can heat up pretty fast if you talk to pilots who are passionate to either one so I don't want to go there (use what you like the best). My own view point is the average RC pilot is going to be best served (performance wise) with good quality digital coreless servos while also saving a few bucks in the process.
If you are wanting the best efficiency, smoothness, and likely lifespan; ante up the coin and go for brushless.