Something to consider with any prop selection is margin against cavitation. In smooth waters, little performance difference may be detectable among the various props mentioned in this thread, but when the going gets tough (punching into rough head seas, for instance...), performance differences will really show up. A standard factory installed three bladed prop on our PSC 40 id just fine under average conditions, but not in rough. 4 blades was the answer. Our 4 bladed Variprop (DF107 at 18" diameter) has been marvelous in that regard - we even have to back off on the power sometimes to keep the crew from getting beat up. The boat is more resistant to slowing after a series of nasty seas and accelerates back to speed much more quickly.
PSC 40 #60