I disagree with everyone
and I will explain why!!!
Think of two power levers the throttle and the elevator.
The throttle will tap into engine power.
The elevator will tap into potential energy inherent in the airframe.
Take the simplest form! remove the engine from the equation ie a glider on a still day and there is only one form of potential energy to tap into to maintain speed and that is the potential energy in the airframe.
In that situation you have one power lever and that is the elevator.
Add an engine and you have two power sources available the airframe and the engine.
So really neither is correct as it is using both sources which is accurate.
Take a light draggy trainer with a low powered engine and pitching for speed becomes more important.
Take a high powered slippery aircraft and pitching for speed becomes less important while power for speed more important.
Really its a play between using two power sources both equally important! Ignore one or follow one rule at your peril!
Take an inexperienced student in a low powered draggy single and the priority has to go to pitch for speed as that will keep the pilot away from a stall situation.
but I repeat you have two throttles to tap into energy why use one????
Ignoring one has the potential for real danger.
Both principals are flawed!
Pace