Perhaps the most obvious is 
to improve precision, which is a function of manufacturing and assembly tolerances, gear tooth surface finish, and the center distance of the tooth mesh. Sound is also affected by gear and housing components in addition to lubricants. In general, expect to pay more for quieter, smoother gears.
Don’t make the mistake of over-specifying the motor. Remember, the insight pinion on the planetary must be able manage the motor’s result torque. What’s more, if you’re using a multi-stage gearhead, the result stage should be strong enough to soak up the developed torque. Certainly, using a better motor than required will require a bigger and more costly gearhead.
Consider current limiting to safely impose limitations on gearbox size. With servomotors, output torque is usually a linear function of current. So besides safeguarding the gearbox, current limiting also defends the electric motor and drive by clipping peak torque, which may be anywhere from 2.5 to 3.5 times continuous torque.
In each planetary stage, five gears are at the same time in mesh. Although it’s impossible to totally get rid of noise from this assembly, there are several ways to reduce it.
As an ancillary benefit, the geometry of planetaries fits the shape of electric motors. Hence the gearhead can be close in diameter to the servomotor, with the result shaft in-line.
Highly rigid (servo grade) gearheads are generally more expensive than lighter duty types. However, for rapid acceleration and deceleration, a servo-grade gearhead may be the only sensible choice. In such applications, the gearhead may be seen as a mechanical spring. The torsional deflection caused by the spring action increases backlash, compounding the consequences of free shaft motion.
Servo-grade gearheads incorporate several construction features to minimize torsional low backlash gearbox stress and deflection. Among the more prevalent are large diameter result shafts and beefed up support for satellite-equipment shafts. Stiff or “rigid” gearheads tend to be the costliest of planetaries.
The kind of bearings supporting the output shaft depends upon the load. High radial or axial loads generally necessitate rolling component bearings. Small planetaries can often manage with low-price sleeve bearings or additional economical types with relatively low axial and radial load ability. For larger and servo-grade gearheads, heavy duty output shaft bearings are usually required.
Like the majority of gears, planetaries make noise. And the faster they run, the louder they get.
Low-backlash planetary gears are also available in lower ratios. Although some types of gears are usually limited by about 50:1 or more, planetary gearheads extend from 3:1 (solitary stage) to 175:1 or more, depending on the amount of stages.