Surge arrester main concept is to utilize the non linear behavior with the rising of voltage level. Say for low voltage it shows mega ohom range resistance but with high rising voltage its resistance drops to 10 ohom range.
As for spark gap which is always constant for any level of voltages, surge arrester is better as it changes its value as per the rising speed of the voltage and its level.
Characteristics of surge arrester
zinc oxide (ZnO) surge arresters act like strongly non-linear resistors.
Under voltage surge conditions, the withstand greatly drops (up to roughly 5 or 10 ohms), thus limiting the voltage in surge arrester terminals and therefore in the equipment for the duration
needed for the lightning current to runoff to the earth.