A DC motor shouldn't "mind" a PWM supply. When you think about it, the rotor windings are switched in and out of circuit on a continuous basis by the commutator as the motor operates. So switching the supply on and off via PWM is very much the same operation.
As others have said, using a rheostat to control speed is both wasteful of power and poor speed control. Assuming a preset rheostat value, as the motor load increases, the current increases, so the voltage loss across the rheostat increases. This leaves less voltage to the motor terminals so the motor tends to slow down. (Motor speed is proportional to voltage)
On the basis that PWM voltage is averaged out, then motor speed is effectively proportional to PWM duty, irrespective of shaft load