Because of the way the human body is made we walk using the back of our legs which become naturally stronger, this is gives us the feeling of greater power during an upstroke these are the most powerful leg muscles. But the down stroke is more efficient in how it delivers weaker muscle force, consider how much easier it is to kick a soccer ball forward than backward. So it will depend on your kicking style, trim, body mass and fins as to which stroke converts more muscle energy into speed/power/velocity. I think that is why most seasoned divers I know speak more of efficient kicking vs. powerful kicking.