Sub-threshold operation is an efficient solution for ultra low power applications. However, it is very sensitive to process variability which can impact the robustness and effective performance of the circuit. On the other hand this sensitivity decreases as we move towards near-threshold operation. In this paper, the impact of variability on sub-threshold and near-threshold circuit performance is investigated through analytical modeling and circuit simulation in a 65 nm industrial low power CMOS process. We show that variability moves the effective minimum energy point towards the near threshold region. Thus, we demonstrate that when variability is taken into account, a complete model that includes the near threshold (moderate inversion) region is necessary in order to correctly model circuit performance around the minimum energy point. Finally, we present the resulting speed-consumption trade-off in a variability-aware analysis of sub-threshold and near-threshold operation