Who do you think would win in a fight: Batman or Superman?
Personally, I think it would go down exactly like it does in The Dark Knight Returns (or The Tower Of Babel storyline from JLA, if you prefer). Batman's greatest power is his mind. If he and Kal-El ever came to blows, Bruce Wayne would be prepared waaaay in advance. There's no way he'd ally himself with someone so powerful unless he had a contingency plan prepared just in case.
The way I see it, this is the ultimate What If.../Elseworlds battle. It's Nietzsche's wet dream: The Will To Power vs. The Übermensch. The unstoppable force vs. the immovable object.
But enough from me. Who do you think would win?