In general, stepper motors are the low-cost solution, both in terms of system cost per channel and engineering effort. DC servo motors offer higher performance on a number of fronts. So you might start by assuming stepper motors, until one of the following considerations pushes you to DC motors:

Positioning accuracy. There are three distinct issues:

1. Positioning with a stepper is open-loop, as seen by the motor controller. In circumstances such as a jam the motor can fail to reach commanded position (”skip steps”). This failure is invisible to the controller, so subsequent positioning will also be wrong. DC servo motors are used with rotary encoders, which ensure reliable feedback of shaft angle.

 2. DC servo motor encoders typically generate 500 pulses per revolution or more. Quadrature decoding provides 4X improvement, ie. to 2000+ counts per revolution. Steppers used in positioning typically have 200 steps per revolution. Microstepping improves this figure, however not to the positioning accuracy achieved by a typical encoder.

3. Once stopped a stepper motor does not stay parked exactly at its nominal step position if disturbance torque is present. Rather it deviates from that position so as to generate restoring torque. This deviation is invisible to the controller. DC servo positioning systems typically also have position error in the presence of a disturbance torque. However this error is visible to the controller and can be addressed via explicit control techniques, such as integral feedback, if necessary.

Shaft speed

Steppers typically operate at hundreds of RPM, depending on supply voltage and other particulars. DC motors can go beyond 10,000 RPM.

Power versus package size

The size of a motor reflects how much torque it can produce, not power. So in general the faster shaft of the DC motor provides greater power for a given size. This is especially true when this power is required in the form of short bursts of high torque, such as to accelerate the inertial load in a high-performance positioning system. Torque demanded of a stepper must be kept safely below its nominal maximum to avoid skipping steps. In contrast, a DC motor can be over-driven to many times its continuous torque rating for short durations.