Both, and a third will emerge. The prophesy states that the "the dragon will have three heads," so maybe Dondarrion? Sure, he's only a minor character so far, but he is the only other one the Lord of Light clearly shows favor towards (dead, what, six times so far?), and he fights with a flaming sword (or was that just a trick to fuck with Der Hund?). Actually, I doubt it will be Dondarrion, but someone else will be included and Bran is actually the most likely since he's going to warg into some dragons eventually.