Depends what you mean by 'speed'? - you can easily change the power to a DC motor using PWM, but without feedback the speed will vary with the load. A stepper motor doesn't really rotate, it moves one step at a time, by stepping it fast enough you give the illusion of smooth rotation - but they are very much slower than a DC motor - perhaps a couple of hundred RPM's?.
Assuming very slow speed is all that's required?, then a stepper would allow you to accurately set the speed - so is probably the best option.