I think "Best 14" is the easiest to manipulate team quality at the moment of Cup drawing, while "average the quality of all teams throughout the season, or past 2-3 seasons" is the hardest to manipulate but it is also difficult to implement. Points of view of supporters of 2 sides must be different.
I am neutral to the suggestion. I don't like the design of Cup because it is unfair. But it provide a different challenge to managers.