A boost regulator (eGo Twist, Provari) will get less time out of a charge by using higher voltage. This is because it must use more power (and thus more current) to convert the lower battery voltage into the higher output voltage.
A buck regulator (Madvapes VV, OKR mods, VMax, anything using 2 batteries in series that reduces the battery voltage to the desired voltage) will get more time out of a charge by using higher voltage. This is due to regulator efficiency, and higher voltage at the same wattage equals less current draw from the batteries. This is especially true for linear buck regulators (Madvapes VV) that shed the excess voltage as heat output; wasted energy. A switching buck regulator uses pulse width modulation to control the output voltage so that much less energy is wasted.