In the air, the 14x6 will make the plane go faster than the 12x6 prop in most cases. This is if you are measuring RPM on the ground and find them the same, as was the original question. That is because the 14x6 will usually become more efficient at flying speeds due to the larger diameter. There is a simple formula that shows this without any engine horsepower considerations. When you measure RPM on the ground, you are only seeing one part of the formula. The airspeed of the model is the other part, and you only get that by flying. This explains why things seem so much better or worse once you fly it.