This is the most important question thrown up by the whole CleganeBowl theory, and probably the hardest to answer. Both are two of the greatest fighters in Westeros, and when they have clashed before they have appeared to be fairly evenly matched. The Mountain in particular is the most feared warrior throughout the land, known for his brutish strength and size that no one can match, and a man who takes great pleasure in inflicting pain and death on others. Sandor, meanwhile, is not quite as feared or as large, but he possesses a burning fury, a hatred for his brother and an overwhelming desire to kill him, which could swing the odds into his favour. Gregor has never been shown to be a major character, so bringing him back to be killed off once again wouldn't be that big of a deal, especially if it fit into the story of Sandor's return. The Hound has becomes one of the most popular and rather tragic figures in the series, an anti-hero that you really root for. He would be deserving of this redemptive arc, and it would make far less sense to bring him back only to be killed off again by his brother. There is the chance that is what George R.R. Martin would want people to think, and he loves to subvert expectations and kill off the heroes and popular characters, so Gregor's win would fit with that line of thinking. However, personally I feel as though this is Sandor's victory. And then he's going to celebrate by eating all of the f***ing chicken.