As an experienced rotary operator, I offer my thoughts.
Jet engines become more efficient as height increases (at least in the height band in which we operate), but rotors become less efficient. So the optimum operating altitude in still air will be different for every engine/rotor combination.
The expression 'in still air' is very significant, in my opinion. The greatest factor in efficiency is, by far, wind velocity. Go high when there is a tail wind component, and low when there is a head wind component. The choice of height in the tail wind option will depend on wind velocities at height, rather then engine/rotor efficiencies.