The piezo can be thought of as a capacitor. If you are driving your output high AND low, then a series resistor and a 50% duty cycle output charges and discharges the "capacitor" evenly. Increase the single resistor and you effectively decrease the current to the piezo in the ON and OFF cycles.
If you use a resistor divider, you are putting one resistor in series with the piezo from the driver pin, and one in parallel with the piezo to ground. This gives you a reduced voltage to the piezo in the ON state, and an reduced impedance during the OFF state. The "drive" is a bit disproportional and may "sound" less amplified on the ON state... but is going to produce a sharper OFF state, albeit one with less voltage. The impedance of the piezo matters here as well... and will react with your resistor divider values, to the point where your Vin*R/(R+R) may not produce exactly half of Vin, but something less.
A pot works well in my opinion, but play with both ways and see what sounds best to you.