Engine Efficieny Question

System

The quest I just completed offers Rare impulse engines. I can't decide between two of them. One "is efficient at low power levels" and the other "efficient at high power levels". I'm not 100% sure what this means. I run an Escort with my engine set at 32/25, so would that count as lower power levels? Or do they mean the impulse speed I'm moving at, because I typically am flying at full speeds.