Two characters. Both are extremely successful businessman, usually depicted (when not dressed for mayhem) in expensive suits. Extremely ambitious. Troubled childhoods. And both, through years of hard work and obsession, have pushed themselves to the limits of what the human body and mind have achieved. Neither has superpowers, and both have complicated relationships with a non-human who, by a quirk of birth (and parentage, which is not the same thing here) has dwarfed everything they have ever done. One is, more often than not, this alien's greatest friend. The other is his most hated enemy.
There's an idea that every great hero, at some point, has to fight himself. Some shadow version who reflects his strengths and especially his weaknesses, which must be overcome to show that he understands himself. Comic books, with their love of presenting themselves as modern mythology, embrace this idea all over the place. Batman, whose obsession and trauma push him to the edge of human endurance, grapples constantly with a laughing clown who is, essentially, the avatar of madness. Green Lantern has the fallen corpsman Sinestro (who has of course spawned an entire shadow-corps of fear-based psychotics). Hell, the Flash has a recurring character literally named "Reverse Flash".
Superman is not immune to this trope. Doomsday, the hulking monster that killed Supes back in the '90s, is Superman's raw force unleavened with mind or compassion. And of course there's Bizarro, the backwards Superman. Bizarro's interesting because he's usually presented as a kind of tragic, ineffectual figure. It's almost as if Superman is so good that even his imperfect, backwards copy absorbs some of that inherent good. And of course, there's the fact that a true, "shadow" Superman would be able to do so much damage that he'd wreck the story-telling universe. Superman, as Grant Morrison has put forward at times, Wins. He always wins, because he is Superman. (Morrison, of course, has actually written a shadow Superman, the bullying, cruel, hateful Ultraman who plays the "Token Evil Teammate" role in Final Crisis: Superman Beyond). Trying to reflect that "winning" quality would be hard to do without a) wrecking the universe) or b) making the villain seem ineffectual (although it would be interesting in the context of the question: Does Superman win because he is perfectly good, or are those two qualities separate? But I digress).
Where was I? Lex and Bruce? Right.
Lex Luthor, Superman's most iconic foe, is NOT a Shadow. Lex is human where Clark is alien, weak where he is strong, utterly self-centered where he is compassionate almost by definition. Lex is humanity denying outside salvation. He is vain and he hates feeling powerless. He is also an extremely intelligent, ambitious, successful businessman. He often bemoans the fact that, if not for Superman, he would rule Metropolis without question, and he makes a compelling case.
Meanwhile, Bruce Wayne has spent years turning his body and mind into the perfect crime-fighting weapons. He has unlimited resources, but has sacrificed family and love any number of times in order to wage his crusade. Unlike Superman, he doesn't fight out of compassion - he fights because he's taking a kind of extended, operatic vengeance on the very idea of crime.
And when compared to Superman, comic writers do everything they can to make him as cool as The Man of Steel, or cooler. When the two fight, Bruce seems to win more than Clark does. Makes sense because a) massively more powerful person winning every time is boring, but b) also because there's a part of us that WANTS to see him win. He's the underdog, and he's us. We want to see a human win, but it can't be Lex Luthor, because he's the bad guy (and the conventions of the medium say that the good guys always win), so Bruce wins for him. It's still a brilliant HUMAN defeating the "perfect" man.
Bruce and Lex are both humanity at the very edges of what they can reach. Superman is comforting because he's "The Man of Tomorrow," the perfect people we could eventually be. He's a dare to be better tomorrow. Batman is an indictment, because he's the man of today. There's a sneer, buried at the heart of the character - if you cared enough, if you were willing to work hard enough, you could have been this good, too. And every time we refuse the help of someone who's "better" than we are, every time we care more about ourselves more than others, every time we get scared that the world has left us powerless and that fear makes us do something selfish or stupid, well, that's Lex. He's the man of today, too. We can pretend he's the man of yesterday, of course, that we've left him and his big green and purple armor behind us. But he's going to keep popping up.
Huh. I appear to have accidentally divided Superman, Batman, and Lex into a sort of pseudo-Freudian trinity. That wasn't the intent when I started this, but it seems inescapable: What we might be someday, what we could be today, what we are right now. Maybe it comes down to this: Lex Luthor wants superpowers, lusts after them, would kill hundreds and thousands to get them. I know the feeling, if not the extreme of the desire. Batman could care less - he has accepted and modified himself, instead of lusting after external power.