Rex, Dark, not gonna lie, at this point I hope that Natsu and Gray are on the opposite ends of the Earth when E.N.D. happens just so that I don't have to deal with the fallout between you two. Though honestly, I can see the two being equal after Gray unveils his secret weapon and then works with Lucy to snap Natsu out of it.
Or Erza could just glare at the two and Natsu and Gray immediately drop out of their respective power-ups and start acting like friends.