Um...
A potentiometer or rheostat is a variable resistor. It works by limiting the resistance in the circuit and therefore the current going through the fan.
A voltage regulator limits the voltage. This may sound like a non-valid point, if the regulation is linear, i.e. you decrease the voltage with the same resistance to give a lower current. However, if you vary the time the voltage is on, but it is on at full strength, you get a much different effect. This is generally referred to as Pulse Width Modulation or PWM and would be nice to have a tutorial about. The benefits of PWN is that the fan will turn even at low modulations because the full voltage is being use to start it. It also is more energy efficient as resistors generate heat, and some reports even show it makes less noise and gives more stable RPMs.