As for the "dual persona" way of looking at things, I personaly still see Anakin and Vader as the same person. To me the point is everyone has that choice wether to be good or bad. Anakin chose to be good up until a certian point, then chose to become bad, then chose to become good again at the very end.
For me the reason why I prefer having Hayden in there is just because to me it visually represents the good man who was Luke's father before he had become Darth Vader. While Shaw was the face he had when he came back to the light, he still visually represents (to me) the twisted wreck he had become as Vader.
But to me it means the same thing as it did before, it's just before when we didn't have the prequels to show us what Anakin looked like before he was put into the suit, we could just assume Shaw with a full head of hair was how he looked before he went into the suit.
That said, I can see why others would prefer to have kept Shaw.