The only real way is to try different props on your boat if you can, as I found for us that the four blade Rev 4 was far better all around:
Mercury props: Al 19P / SS 19P / SS 21P / SS Rev4 19P
@ 3000 rpm: 24.7 / 23 / 26 / 26.6 kn
@ Wot: 36.1 / 40 / 39 / 39.7 kn
The basic 19 inch pitch aluminium 3 blade was surprisingly better than a 19P stainless version at cruising speed, but the 4 bladed was best. Nothing different at top end with the stainless props better than the stock aluminium. Acceleration and general response was noticeably best on our boat with the Mercury 4 bladed Rev 4.
The 21 pitch did reduce the top revs slightly.
The boat is an inboard mercruiser (190hp) regal 1900 bow rider with a stepped hull, which causes more cavitation challenges for the prop to deal with, but no issues at all at any speed with the Rev 4. One test with a 3 bladed stainless prop without the hub plugs in it wouldn't even get on the plane!
So we were really pleased with the chance to try these different combinations.