Right now im running 4 blades on my twin F200, thinking of going to either 15x17 mercury mirage or 15x19 Yamaha SWS 3 blade. anyone have opinions?

That is the most difficult questions in engine performance.
You HAVE to stay in the manufacturer's recommended RPM.

Not over, especially not under.

Smaller diameter, higher pitch OR
Bigger diameter, lower pitch.
3 blade OR 4 blade.
Coated OR uncoated.
Then which manufacturer ????? (They are not all the same even for
the same spec prop.)

And all the combinations of the above.
It goes on and on.........

Its really what works, there are all kinds of calculations and formulas, but when you come down to it you have to put them on the boat and run it.