OK my take on this. Cap will win the majority but, like in every boxing fight, the underdog has a puncher's chance.
The way I see it, this ain't a random match. This is a prepped and sanctioned match so that means they would know each other somehow. Cap, knowing he is the superior fighter would be the pressure fighter/swarmer. I'm not saying he'd be a slugger. That's just plain stupid. But he would best capitalize his superior physicality with overwhelming his opponents with his power and speed. Think a more accurate, more tactical, harder punching, better chin Tyson in his prime.
Batman on the other hand would most likely take the defensive road knowing he's at a disadvantage, timing his shots, ducking, weaving, using as few punches as possible. Bruce is not an idiot who would want to duke it out with a superior enemy. Probably a boxer/puncher with some pretty good counter punching. Let's say something like Ali or Holyfield. Ok, I know Evander has beaten Tyson and a lot of people would not really accept Tyson beating Ali. But how could I say that Cap still wins? Well he is superhuman. And I'm pretty sure he's smarter than Mike.
So there. Best way I see it, both can knockout each other given the chance. Steve would and could penetrate Bruce's defenses with enough punching. I can see that as a possibility but not always. And yes, Batman, inferior as he may, can KO Steve if the situation, setup and the targeting is right in his favor. But in the majority of the fights, if we take this as a 12-rounder, I think it will be a win for Cap via UD. Yes, I do believe Batman can survive Cap's barrages. But the fact that he would not be the aggressor (the only way he'd really survive) means he'd lose in points.