I would use an electronise speed controller every time, they are faultless, to save a bob or two you can buy them in kit form as well, they have never let me down, I have used allsorts of other ones but for tug work they are the best. I drive a bosch car fan motor on a 15amp one at 12 volts, my smaller tug has a 900 size motor with a 15 amp controller as well, you get no high pitchwhining or strange noises with them at all.
Andy