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Like Phil said it depends on your running conditions/weight.. If the 15 pitch doesn't let your OB come up to max RPM at WOT then you need less pitch, If the 13 pitch makes your OB run over max RPM at WOT you need a bigger pitch.
Hard to figure out with our current weather here in the Mid-West.
Good luck and hopefully your dealer lets you try each different pitch to determine your best set-up.