An esc is better than an msc because an esc pulses power to the motor very quickly which uses up only as much energy as you want, an msc changes speed by running power through resistors which suck up a lot of power. It was always explained to me that with an msc your runtimes will be the same no matter how fast your going, and that that runtime is equal to going full throttle constantly with an esc. Not sure if thats exactly true but it explains the concept.