It depends on why you are measuring the current. A DC current reading will be a good estimate you can use to size the motor for a torque rating. If you want to measure current in order to determine whether or not your motor/gearmotor is sized adequately (i.e. you want to load test your motor/gearmotor by measuring current and calculating torque from measured current), use a DC current reading. An AC current reading will be essentially the same as the true RMS current through the motor. This is a better measure of the current that is flowing through the motor. This measurement will be more useful if you want to determine the amount of dissipated heat the motor is producing.