what RPM are you turning at WOT?
generally speaking, the smaller prop, more low end & less top end. larger prop is the opposite.
However since it takes more HP to turn the larger prop you are not guaranteed more MPH.
I suggest borrowing a 24 and trying it before you buy one. Try it with your usual load of passengers and beer.
You might pick up some MPH but also lose some low end. You will probably drop 200-300 rpm.