Comment by bell-cot

They're both - though Intel has mostly talked up the power efficiency.

For CPU's, those two types of efficiency are closely related. Omitted transistors (in an E core design) neither take up die space, nor consume power. And CPU cooling systems are ultimately measured by how many watts of heat they can remove from each unit of die area - so fewer watts from a smaller core. (That's at a given temperature difference, etc. But your die will die if any part of it gets too hot. And revving up the CPU cooling fan is generally not preferred.)