The first thing to care about is your price factor: the same power/torque motor, how much you are.
Performance: steady and accurate. This is almost a basic requirement. Accelerated performance, the same size of the load accelerates to the same speed, who can achieve faster acceleration at a more economical cost.
Speed control range: Whether to support a wide speed range.
Control accuracy: How much can be achieved.
Reliability: Working long hours, there will be problems, how many years can be used. How high is the temperature rise of the motor.