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I think the longer vs shorter might have more to do with those "powerful strokes" being usually done with high angle strokes. Hence the shorter paddle works out better.
My WW paddle is 194, while my touring paddle is 215! Granted, the WW paddle has a larger surface and I wouldn't be able to drive it at 215 length. But if I want to paddle all out on my touring paddle, I'd prefer to use a higher angle stroke and would have liked to have it at 205 instead of 215.
Can't say rough vs flat water is that clear of a difference though...