Here some test to compare:
RECKONING:
Def stance:
Your Flashing Arc IV slashes Straw Man for 284.
Your Flashing Arc IV slashes Straw Man for 300.
Your Flashing Arc IV slashes Straw Man for 270.
Your Reckoning hits Straw Man for 180 slashing damage.
Frenzy stance:
Your Flashing Arc IV slashes Straw Man for 369.
Your Flashing Arc IV slashes Straw Man for 391.
Your Flashing Arc IV slashes Straw Man for 351.
Your Reckoning hits Straw Man for 794 slashing damage.
Damage boost is about +340%
PRIME INITIATIVE:
Def stance:
Your Prime Initiative hits Straw Man for 612 crushing damage.
Frenzy stance:
Your Prime Initiative hits Straw Man for 993 crushing damage.
Damage boost is about +60%.