Im not sure if I agree with the great Mr Stu (Sorry Stu!)on this one. You need to off set the extra weight of another speed controler against a simple on/off switch. He is quite correct about a momentery dead short (although its not a true dead short as the motor does have an inpeadence all be it a low one and the load reduces very rapidly as soon as the motor starts to turn) We have tried it both ways and experience shows that an on/off switch wins every time. We pull (in theory) around 110A on start up but that is for less that 1/4 second and if you do the maths that produces so little extra heat as to make no diference over all. What does matter is how hard the motor has to work to keep it all spinning. That of course is down to things like gear ratio, bearing and drive train resistance and overall motor power.(make sure your motor is powerfull enough for what you want it to do)
A speed controler will be a little kinder to your motor, but you will increase the spin-up time and you have the extra weight of the controler. So you need to ballence all this against a faster spin-up time and a hot motor!
Your choice!