ORIGINAL: Sport_Pilot
A differance in prop speed from one prop to another is not an indication of efficieny. It may be a differance in pitch and total area. At any rate if it were from a change in efficiency then the engine would slow down not speed up, because the prop is moving more air and doing more work.
The difference between any two props will (almost) always reflect a difference in efficiency. Pitch and total prop area affect how much air and how well the air is moved. One prop will always be more efficient than another, based on it's design, unless they are identical.
Each manufacturer strives to produce a prop that is the most efficient, i.e., moves the aircraft the most, with the least power consumed. RPM is one indicator; thrust is another, but with only a fixed amount of HP to play with, you have to play these two goals (RPM vs. Thrust) against each another. A flatter prop will spin faster, but generate less thrust wth every revolution. Whether this ends up generating more thrust overall depends on the torque curve of that particular engine. Also, a heavier prop will be slower to spool up, but similar in constant speed performance. Everything affects efficiency.
Jack
Small print - I'm a Mechanical Enginerd, relatively new to this sport and aircraft in general, possibly speaking out my butt on this issue, but I believe I'm correct.