Some factors to consider in maximizing your application systems performance:
Torque Multiplication
Radial Loading
Noise
Speed Reduction
Axial Loading
Inertia Matching
Spur gearheads will suit most needs in relatively low-torque applications. However, spur configurations have higher backlash and are usually less efficient than planetary types of similar construction. For constant velocity and unidirectional applications where backlash is less of a concern, spur gearheads are ideal.