This paper shows that for a time-independent specific tax and a time-independent ad valorem tax that induce the same competitive equilibrium in the Hotelling model of resource extraction, the ad valorem tax yields a higher level of discounted tax revenues than the specific tax. Moreover, given the same level of discounted tax revenues, the ad valorem tax also yields a higher level of social welfare. Finally, for the time-dependent schedules of optimal ad valorem tax and optimal specific tax, we show that when appropriately set, they are equivalent in implementing the dynamic social optimum and providing the same discounted tax revenues.