Just thinking out loud here. I agree with you that the thinner leading edge would provide less 'resistance' because of the lesser frontal area. But I do believe a thicker rounded edge would be more forgiving and allow for better flow efficiency if the vane was not accurately aligned with the air flow direction.
Very well be more forgiving on alignment with a blunter front end. Makes sense to my feeble mind. Considering the tornado that's going on in the plenum, I'm sure that it is a very dynamic situation in front of every port entrance.