Pitch v diameter - Prop question

[Apollo]

Question for the ones in the know:

What is the effect of decreasing the diameter of a prop and increasing the pitch?
An example is my 20hp merc has the option of a 10 3/8" 13P or a 10" 15P.

While I am at it- what is the reasoning for going to more blades? 2->3->4?

Thanks
Steve

[krazyfisher]

rule of thumb that I have worked by is .25" in dia = 2" in pitch so the 10 3/8" is the bigger.... I think