Beating it in the box office? Tricky. I mean, this IS Batman Vs. Superman, a movie fans have been wanting for gods know how long.
But critically? I think Civil War has a good chance of being the better movie.
Yeah, I think just being Batman v Superman is giving that movie a big advantage. But, Civil War is basically Captain America vs Iron Man: The movie, and that's not insignificant. I think Marvel will easily have the more successful movie critically, but it will probably be a fight box office wise. Still, even with the novelty of seeing Batman and Superman together, I think Marvel at least has a 50/50 chance of beating it in the box office.
It's going to be close, that's for sure. And you're right, it is basicly Cap vs Iron Man, something Marvel will market for all it's worth.
I guess we'll have to wait and see, it's impossible to predict which one will 'win'.