not always true in comparing CS either as MP/RPM/DA provides you the Shaft Horsepower from the engine. What makes the plane go is Thrust Horsepower which is shaft power * the prop efficiency. Prop efficiency is a very tricky variable in its own right and is effected by RPM/Dia/airfoil and DA. So simply setting engine power will not provide a valid comparison between props since you can set the same horsepower in a variety of MP/RPM combinations all of which result in different prop efficiencies.
to compare you need to know the efficiency curve for the prop you are flying and map the maximum to engine RPM for a given DA. Use this RPM to map to to the MP that provides maximum SHP. This will be the engine SHP for a given DA at which the propellor is most efficient. The prop that allows the highest SHP at maximum efficiency would be the prop with the most top end performance.
it is not an easy task to TEST prop efficiency as installed since to do so requires many data points that are easily affected by many variables. To do so thoroughly you'll need to know how to normalize the data you collect to isolate vairables such as change in weight (due to fuel burn), DA, etc..
However you can get a quick check by picking a mid power Hp for your engine.. say 150Hp and using your engine charts to find at least 4 different MP/RPM combinations that equal 150Hp (don't worry about the silly rule of never having your MP higher than your RPM - if the combination is on the chart it is a safe to operate the engine there) then go out and fly those 4 combinations while recording IAS for each. The more quickly you work through these the better your data since your weight is decreasing. Plot the data on a chart of IAS vs RPM to find the max prop efficiency RPM at 150Hp. Repeat this process for other power settings above and below 150 and plot on same chart. Now draw a line through the peaks of all those curves and this will give you an idea of how your prop efficiency changes with power and where your prop is most efficient for the flight regime you normally fly. Then you can make up a Mp/RPM chart that you bring into the cockpit so that rather than follow the overly simplistic method of flying 25/25 for cruise you may discover that flying 26"/2450 is more efficient giving you more speed for same fuel flow (which is really the measure of efficiency you are after). The closer you conduct your test with the plane in same configuration (CG,GW, etc) and test day conditions (DA) the more accurate you'll be...