Xerneas and Yveltal have a life/death motif which could be played as a good/evil thing. Does this mean rather than the two games being what amount to palette swaps they'll actually have significant differences plot or structure wise?

I mean think about the climax where you battle the box legendary. In Y you go to Yveltal to defeat him and stop his destruction, in X you...go fight Xerneas to stop his tender love and care for life on earth...yeah ok that totally makes sense

So will X and Y actually be fairly different because of this?