There seems to be some mixing of terms in this debate. Alliot talks of stability with a 3-blade vs. a 2 blade. The only reason the 3-blade would have "stability" would be more gyroscopic effect from the added weight of the 3rd blade. The downside to this is you will lose manueverability and some power.

Comparing a 14X6 to a 13X8 is a poor comparision. The greater pitch on the 13X8 is responsible for the increase in speed. Try comparing a 14X8 with the 13x8.

Now for efficiency. Don't confuse efficiency with speed. Efficiency is the measure of how well the propellor is converting the engines power into thrust. If you turn a 2-blade and a 3 blade the 2-blade will generally have more efficiency than the 3-blade because the blades are better able to get out of the disturbed air from the previous blade's passing. With that said, you'r 2-blade propellor may very well loose some of those gains because of the higher tip speeds due to its larger diameter. The tips are thrashing the air more and that does nothing for you.

Where the performance curves lie and the relative losses from multiple blades and the tip speed issue I don't know. It is generally accepted in full scale avaition that you add blades to reduce diameter. With that said, full scale engines turned props much slower than models. So the loses from increases in tip speed for models can come into play.

I know I haven't really answered this question, but I hope this clarifies some of the physics involved. The best way to select a prop for a model remains trial and error. If you find a multi-blade prop works better then go for it. Besides, they do look kind of cool.