People argue. Pure and simple, they just do. It comes down to where someone was born, with what genes, and how they had the world introduced to them. These things lead to a persons paradigm, the lens through which they see the world. When a person is challenged, it is their lens being questioned, their perception of reality which is implied to be at fault. This is why in almost any situation, no matter how nuanced, people will find something to passionately argue about.
Comic books. Comic books came about following pulp novels in the late thirties, and became highly successful as a cheap, disposable, and recurrent form of entertainment. They found true success with Super Heroes: Awe inspiring individuals, a new pantheon of Gods, each representing their own ideals, but affirming that Good triumphs over evil. As any American tale, competition existed within the format, as multiple publishers popped up, disappeared, reappeared, renamed themselves, and left us today with two primary comic companies (At least where superheros are concerned, for all intents of this article, I have chosen to leave out other publishers).
From 1934 through 1937, National Allied Productions produced a few series of comics, most notably Action Comics and Detective Comics, each featuring the debuts of Superman and Batman respectively. DC invented the Superhero, and too invented the spectrum that all heroes would fall within in terms of power and humanity. The company later took on the name Detective Comics, or more colloquially DC.
Timely Comics came about in 1939 and ran a number of successful titles as well, including Captain America in 1940. But true success came two name changes later, as Timely became Atlas, and Atlas became Marvel in the early 1960s. Kirby, Ditko, and Lee brought new and exciting stories of Spiderman and the Fantastic 4 into the public eye, establishing a firm place in the world of comics.
Today, characters of each company are incredibly prevalent in popular culture. This is due in a large part to the fact that Superhero Movies of succeeded so well as a genre. A result of this, however, is that the argument of Marvel vs. DC has extended beyond the devotee comic readers, and too the general public. It is almost humorous how vehemently people will defend their side, but it is understandable.
Part of what makes these characters so popular, and makes people care so much about them is the ideals they represent, and the role they have played in people's lives. When someone says Batman can beat Superman, it is less about the physicality of the fight, and more about the idea that a man can stand up to a god. That we as humans are not, in this world of super beings, out of control. That strategy will always win out against brawn. Now whether either would really win is irrelevant, it's about the ideas, and what they mean to the individual.
Ideas are all that matter to humans. We interact with the physical, but everything is representative and emotive of something else. The differing ideals of the individual lead them to their stance, so again, to question their stance is to question their ideals. The truth is, neither Marvel nor DC is better than the other, because Marvel and DC simply represent different ideologies.
Marvel is tales of wonder, but relatability. It gives us people, with true problems that we have felt ourselves, rise up over these problems, defeat a villain that is representative of these on some level, and lets the hero grow as a person. Spiderman is perhaps the greatest example of this. He isn't an alien. He isn't a billionaire. He's a highschool kid, who has this power thrust upon him, and has to do what he thinks is right, protecting his world while dealing with his own stresses. It is the modern heroes tale, one of the most successful lines of story telling the world's known. It is also incredibly useful on the big screen, and has really opened up the market of Superhero Movies. The Movies are fun, witty, relatable, and produced with a high level a quality, all without taking themselves too seriously.
DC changed drastically in the 80s. With more mature writers, the storylines became darker, sometimes without the hero winning at the end, or even leaving the reader to question who the hero was. Tales like Watchmen and The Dark Knight Returns pull apart the idea of a world in which Superheroes exist. In each, there are protagonists, but no real good guys. DC branched into the philosophical background of what it means to be a hero, and what the actions of these heroes imply. Is Batman a fascist? How many deaths away is Superman from being a dictator? In these more mature storylines, DC has found it's niche, a niche which has not translated too well to film, but which is incomparable on television. And not even the modern shows. Animated DC films, all going back to the 1992 Batman the Animated Series to the Justice League of the early 2000s, give us a world with Superheroes in it. A world in which actions had consequences, and politics were debated on a show which was primarily directed toward children.
Now, don't get me wrong, neither publisher is wholly set in their ways. In fact, many writers and artists work on both sides of the line. Marvel has some storylines such as Civil War, which questions the political ramifications of Superheroes hiding from the public, and God Loves, Man Kills, which points out discrimination using characters we love and care about, commenting on our society. DC has their share of relatable tales, such as Flashpoint, which expresses defeating inner demons and an external enemy, accepting the things one can't change.
Each company has it's niche, and each is wonderful in it's own right, but neither is objectively better than the other, unless an objective standard is set. If you are and individual whose paradigm dictates that they enjoy the relatable tales of discovery and triumph, you may lean towards Marvel. If you find yourself more concerned with the implications of a world with superpowered beings within it, you may lean more towards DC. The point is, there should no longer be an argument of which one is better than the other, simply the championing of each for the great content the have put out in their time, and our chance to engage with that on today's many levels of media.