I'm a well read grad student who's bluntly honest about all things, although I try to be most honest about myself.
Batman and Bane are both orphaned, both have incredible wills and both could have been either the Bat or the villain. Except that, arguably, Bruce Wayne is good - and Bane is not. Except it's not that simple. While Bruce was rich, and had a devoted caretaker, Bane was not; he was raised in prison, forced to eat raw fish, and had to fight not to drown when his cell flooded. He wasn't raised with love after his imprisoned mother died, and in Santa Prisca - where Bane grew up, and considered the worst place on Earth by much of the DC universe - he had to depend on himself.
Contrast tis to Wayne's childhood, loved and pampered while his parents were alive, and taken care of when they died. Alfred was a father figure, who loved Bruce fiercely, and all his needs were seen to. While Bruce pushed himself to become so strong in body and mind, he was never unwillingly in this position. (While the panels of Bruce, fearful of the dark right after he's orphaned, are incredibly painful to read, the panel of Bane eating raw fish, and talking to his mommy, telling her he doesn't like it, is gut wrenching. That scene keeps coming up because it's haunting me. It's haunting.)
Would Wayne and Bane have turned out to be similar if their situations were reversed? This comic teases that possibility by showing the parallels, and differences, in their lives. For people who had such similar, and traumatic, experiences, they also had some incredibly divergent stories, too.
And the brilliance of this comic isn't in the fights, but in the way the action is a prop for these characters, propelling them onward, and revealing the darkest possibilities in between the lines. More than that, in Secret Six, Bane is shown to be more than a power hungry monster: he finds a daughter in Scandal Savage, and he doesn't show affection openly, and he's hard on her all the time, but he clearly cares for her a great deal. He's not a sociopath, but rather he hasn't been shown kindness by anyone, and all life has taught him is that power means a better chance of survival.
I don't think either character - Batman or Bane - would be the same if they'd swapped lives. That would suggest that genetics, and one's personality, was completely based on nurture rather than nature. I believe that not only is this untrue - I believe in a fair mix of nature and nurture - but that it would make for a fairly boring story. It would still be exactly the same story King's already telling, but with the characters switched. (Marvel's What Ifs, as well as IDWs Deviations, show that this doesn't work: if you have a different story with the same ending, it feels derivative.) But again, King's genius here is that he doesn't tell that story, and doesn't try. He instead draws the parallels for you, and lets you question what it would be like if the histories of the characters were swapped.
Love, love, love.