At large sizes, synchronous motors are much more efficient than stepper motors, so they use much less power.
At small sizes, the synchronous motors must use permanent magnets, which increase their cost and they have windings that are more difficult to make and the difficulty increases with the smallness of the motor.
The electronic drive of a synchronous motor is more expensive, because it must generate sinusoidal currents, not rectangular currents.
At small sizes, a synchronous motor may have a lower torque than a stepper motor , so it might need extra gears, which would increase the cost.
As long as it is still cost-effective to manufacture a synchronous motor, it will always have a better efficiency and a lower power consumption than a stepper motor. The reason why stepper motors are preferred is that at very small sizes they can be much cheaper, especially when including the cost of all associated electronic and mechanical components.
