Gun Kata is solely based on using firearms in combat, so in order for this duel to occur, Preston would have to be able to use a gun...therefore...he wins!
sidenote - The Bride would however win in a battle of the sexiness...which I think is more important anyway
The Bride she'll chop his arm of before he can shot her just like she chopped of Sophies, then she'll stab him where it hurts most, and therefore automatically win, hands down!
John Preston without a doubt. That was the only film I ever saw where the top fighter never lost. He dominated and I loooooved that aspect of the film. She would go down quickly and hard.