Now there's a preview with the two beating the crap out of each other. Who do I root for? Is it gray vs. gray—superheroes with this gray morality crap, where everyone's a hero until an agenda changes that? Is there a villain instigating the fight so that, halfway into the movie they're buddies again, so the "Vs." in the title is really an ampersand's secret identity?
To me, the answers are all in the trailers. I mean this movie takes place in the Nolan Batman universe, which includes Man of Steel. They're creating a shared universe like Marvel, so none of the other movies outside of those involving Nolan are cannon. If you've seen those movies, you'd know who these characters are generally within that context.