ORIGINAL: ptulmer
Dave, keep in mind that a harder working prop may spin fewer RPMs. That's good testing, but doesn't show the effectiveness of the setup on a particular airframe. For instance, the Cox 5x3 has been one of my better props on a fairly heavy (17oz) delta. The 6x3 MA would barely get into the air. I might as well have been swinging a popsicle stick!
Good on you Dave, experimentin' is lots of fun, isn't it? I agree with ptulmer, you may be surprised at how well a prop pulls in the air despite the low RPMs on the ground. Just look at an APC 6 X 3 compared to a Cox 6 X 3. Way more blade area on the Cox and it pulls far better than the APC despite the latter doing a full 1.5K better on the bench. However, this all depends, even, on the plane you're pulling. Best to just try a variety of props in the air. My favourite for sport flying is the Cox 6 X 3 black, rubber ducky.