using pwm will make your robot faster than by varying the voltage. using pwm you are supplying the motor with full voltage, and the ability to draw full current that it demands, there for your motor will be developing maximum torque at any given speed.
You are just varying the time that the motor has power applied, but except at very low speeds, you will not see any jerkiness - and when you want it to speed up, it will accellerate much faster than it does just now, as it has much more available torque.