What ORP sensor should I use - platinum or gold?

Oxidation-Reduction Potential sensors have either platinum or gold sensing portions due to the low reactivity of the metals.

Platinum sensors are usually used in oxidizing reactions (above 500mV) such as pools & spas and for testing municipal drinking water. 

Gold sensors are used in reducing environments (below 500mV) such as galvanic applications and in the mining industry (cyanide).