Judging between the The Howling Wind and The Wailing Wind, I believe that The Howling Wind has a higher drop rate of Vortical Pinwheels.
You get two types of chest (red and gold) to have the treasure. Plus, there's a higher probability (in 1/3 or 30% ~ 33%) for the red chest to drop the leaf. At least one red chest seems to be a guarantee for the hosting player of the co-op battle.
From a casual test (not nearly enough for a reliable sample size and didn't use any kind of drop buff), these were my results:
Howling Wind only gave me 1 Vortical Pinwheel out of 10 battles, from a gold chest. None of the red chests gave me Pinwheels.
I tested both Talon Strike and Wailing Wind; IIRC all the Hard Stages have the same drop rates, but obviously Talon Strike has less HP than Wailing Wind and thus takes less time to fight.
From Talon Strike, I got a total of 4 Pinwheels in 10 battles across gold and red chests; 3 from red chests, 1 from a gold chest. In 10 Wailing Wind Battles, I got 5 Pinwheels, 4 from red chests and 1 from gold chests.
Due to the small sample size, it's likely that Talon Strike and Wailing Wind are about equal in drop rate and so for farming, Talon Strike would in fact be better. However, even from this small test, it seems quite likely that the Hard stages are better for farming Pinwheels (or any of those gold chest treasures) than Normal stages.