Simple PWM vs Buck Topology for Motor Speed Control
Neither circuit actually controls motor speed, only the voltage sent to the motor. However the speed of a PMDC motor is proportional to supply voltage, so varying the motor voltage is commonly called 'speed control' even though it doesn't actually set the speed directly. 

The main difference between the two methods is that the LM3485 produces a regulated output voltage, whereas with simple PWM switching the motor just receives a proportion of the supply voltage. If the supply voltage is not stable then the LM3485 will be able to hold the motor speed more constant. 

The main disadvantage of using a buck regulator is increased loss in the inductor. This becomes a problem with high power motors, because you need a physically large inductor to keep losses down. In a simple PWM circuit the motor's internal inductance is used to smooth out current flow. This causes increased power loss in the motor, but this loss is usually small relative to other motor losses (provided that the PWM frequency is high enough - not always achieved in practice!).