If you intend to force constant current through the motor via feedback, you will get the poorest speed regulation possible. In order to maintain constant speed, a motor's current has to increase as load increases. Constant voltage has better regulation, but as mechie said, you really need tachometer feedback. If you can add a tachometer wheel or some other device that will give you multiple pulses (the more the better) per revolution, and make this the sensor for your feedback loop, you will get better response time to load changes, especially if you are running at low RPMs.
Ron