How would it spoil the storyline exactly? Their fight has been foreshadowed since part I. They may not fight to the death but they will most likely fight for Position of Hokage Like Danzo and Hiruzen did.According to gamamaru, naruto vs sasuke is going to happen soon. But there is a high possibility that hagoromo is going to TnJ sasuke and naruto. Personally i don't want them to fight either because it would spoil the storyline. To answer your question they are going to fight after the war most likely