I voted no preference. The problem is, this is like having a new actor/actress brought in to play an established role that has been created an developed by a previous actor/actress. You associate that character with the former actor/actress and almost resent the new one for trying to take over. Because of this, I think some people might not be able to accept Paul Kidby's work on it's own merits.
With some people poor Paul will never win, his work is not the same as Kirby's, so in their eyes, it's not 'proper' Discworld art, yet if he attempted to paint in a similar style, he would also be criticised.
Both are and were good illustrators, both have a different approach and style. Don't forget, illustration would be incredibly boring if everyone worked in the same way, producing the same work.
Lets also not forget that Josh Kirby's long career began way before the Discworld, in the 1950's and included film poster art (Return of the Jedi and Monty Python's Life of Brian amongst them) and book covers for war, cowboy, romance and adventure novels as well as SF. (He created the cover for the first Pan edition of Ian Fleming's 'Moonraker', 1956.) His personal catalogue of published book covers extends to over 400.