Marvel and DC Comics are obviously the two major comic publishers. Are there any differences in their philosophies on comics published? Marvel seems to have much more fantastic superheroes than DC, for example. Is there a conscious segregation between the two?
The discussion of the difference between the philosophy of DC's comic creations and Marvel's isn't nearly as wide as it once was. The main difference is the approach to the characters and their perspectives. Unfortunately, both mythologies have become a bit tarnished and it is more about a profit motive than maintaining their previous ideologies.
Since most of the previous creators of those mythologies have died or choose to remain silent, the two groups appear more and more alike as new talents and leaderships take over how the characters, look, behavior and thinking. So I will be looking at them more historically (as a long-time reader) than as a modern comic consumer who may not see any appreciable difference between the two groups. (And they wouldn't be wrong...)
The DC Way
DC started its hero-making defining its heroes as mythic beings who worked as paragons of Humanity, literally the best of what a Man could be. Superman, Batman, Wonder Woman were all paragons of virtue; strength, will-power or personal fortitude.
DC's Batman shows the indomitable spirit of Man, Wonder Woman shows that peace is desirable but may often have to be fought for. Superman shows the innate goodness of hard work, moderation in all things, restraint and humility even when blessed with phenomenal power. He is a god aspiring for humanity.
Their heroes covered every elemental region and often replicated gods of myth. Aquaman (Posidon), the Flash (Hermes), the Martian Manhunter (Proteus), each of these heroes had mythic counterparts though you may not recognize them immediately. Even their non-powered heroes were legendary in their abilities, e.g. Green Arrow or Batman both physical specimens whose skills allowed them to hang with the gods as legendary heroes like Hercules and Perseus.
DC's heroes can be likened unto gods who have come to Earth (in some cases literally e.g. Superman). They exist to be the exemplars of how great humanity can be when we work together toward common goals and their heroes promote the future of Humanity.
This same mythic nature works against DC when they try to create new heroes who are not as mythic and this may explain why most of their new creations simply never gain the ground their iconic originals did.
The Marvel Way
Marvel's approach was somewhat different. Where DC showed gods in men's clothing, Marvel wanted to elevate Men (and Women) to a god-like stature.
Frail mortals such as Peter Parker and Dr. Banner were suddenly endowed with god-like capacities but with all-too human frailties. Anger, rage, indifference, contempt, mortal weaknesses which gave Marvel's heroes a more human, more understandable every-man quality.
Marvel tried to create heroes who often acquired superheroic capacities, the Fantastic Four being the perfect example. Astronauts, scientists, pilots, stuntmen the F.F. were already some of the best at what they did, so when they were bathed in the cosmic storm which gives them their powers, they simply experienced an apotheosis, an extending of their heroic status into a god-like state.
Marvel further expounds on that god-like state when they have the Mighty Thor (who is believed to be a god) come to Earth to battle the Fantastic Four early in their career. Though they are beaten, Thor admits they were far more than mere mortals. This is the evolution of most of Marvel's early character designs, including Daredevil (enhanced senses liked to extraordinary fighting capacity), Luke Cage (superhuman strength and steel hard skin) and Doctor Strange (accomplished surgeon turned Sorcerer Supreme).
Marvel also played with the other half of the human equation, the desire for normality. For those beings who were born with capacities that made them more than Human, Humanity is cruel and unforgiving. Two examples of groups seeking acceptance and never finding it are the Inhumans and the mutant (also known as Homo Superior).
The Inhumans, subjects of experimentation from millennia ago, each member was different on the outside than any other member, but they were a warm and loving people (for the most part) extraordinary beings simply wanting what ordinary people wanted, acceptance and respect.
The X-men and other mutants blessed with superhuman abilities were not revered but reviled, by humanity who feared this embedded potential for change right within their very DNA. Marvel also paid homage to the self-made men such as Hank Pym and Tony Stark whose genius allowed them to supersede their humanity using technology.
Marvel's most iconic heroes have been consistently able to be related to by a wider audience because their primary issues are very human, seasoned with a superhuman sauce. Tony Stark has translated well to the silver screen as a wise-cracking, know-it-all, ever so technically smart and yet in some ways so socially inept. Spider-Man for all of his ability still is a young man trying to find himself and adjust to the idea of great power and its inherent responsibility.
Despite their differences, both companies have successfully managed to tread the ground where most humans find themselves needing guidance and looking to mythology to find the answers. What do I do when I find myself in a position of power and authority? How do I conduct myself? How do I lead effectively? How do I set a good example? Is this the best I can be?
You say that DC has more plausible characters, but actually, Marvel Comics has tried to make most of their superheroes more realistic than DC. Stan Lee has said in a few interviews that when he created most of his comics, he wanted his fans to think that it could happen to them, so he made situations more realistic. Like a radio active spider bite, or an accidental dose of gamma rays, or they were all just born that way, etc. DC comics are far more far fetched because half of their heroes are aliens or all the superheroes are super geniuses like Batman, Atom or Mr Terrific.
But, because the origin stories are more plausible, the writers have already gained your trust and can create more elaborate or fantastical stories and you'd still think they could happen. With DC comics, once we accept that the superheroes are super because they aren't from this planet or they are richer than God, then the stories stay at that level of fantasy and the reader never really sees how fantastic the characters are, when in comparison to other comics, are much more elaborate or fictitious.
Stan Lee himself answered this: he grew up with DC characters like Superman and didnt like the fact their "powers" came from no where. Superman just jumps up and flies, etc. His idea was to create characters who had a plausible reason (albeit based in fantasy) for gaining these powers. The idea of extreme genetic mutation (X-men), bitten by radioactive spider (Peter Parker), etc.
DC characters are basically nut cases with strange fetishes and illogical powers.
It is not too much, but the philosophies of Marvel and DC comics are simple. However, the weight behind them carries more than one would think. When you say "Fantastic Heroes" I am guessing you mean the way the Marvel Heroes achieve their objectives without casualty and/or sacrifice. As both a fan of both franchises I can speculate that DC has more realism than Marvel. Although the heroes in DC are more far-fetched as to obtaining their powers and gifts, they encounter mid-life crises and tragedy as much as the average Police Officer, Fire Fighter, and Military Personnel, or in other words: to incur the wrath and hate of others just by indulging and following your beliefs. However, DC shows the full extent of the consequences for fighting for what you believe in, both in terms of philosophy and mental stability. Marvel on the contrary, takes a different approach. When seeing tragedy or an impossible situation approaching, Marvel Heroes always "Try to find a better way" as to avoid the oncoming event, prompting that no matter the situation may be, it can always work out or there's a solution without needed sacrifice. In conclusion, both Comic Franchise's philosophies have their pros and cons. For Marvel: Always believing that heroes can prevent every disaster is foolish. As individuals who shoulder the burden of billions, they must realize the reality of saving everyone is impossible. For DC: The grim reality of being a hero can be devastating, even for the most adamant. But despite the difficulty of being a hero, people will always be hoping and building towards a better future with or without the help of heroes. And that's all I can say, hope it helps
At heart, it's an issue of identity.
DC heroes are superheroes with secret identities. It's not that Clark Kent is Superman---it's that Superman disguises himself as Clark Kent. Green Lantern disguises himself as Hal Jordan (or John Stewart, etc.). This isn't necessarily bad, just think Batman: Batman has become the identity, and Bruce Wayne has become the disguise (a concept that has generated some of the greatest stories in comic history, with layers of psychological complexity). To me Batman will always be the greatest hero DC has to offer precisely b/c the character is delves deepest with identity issues. It's yet another reason why Batman/Joker is the greatest DC rivalry: Joker's original identity, too, has all but disappeared. They are two people who have transferred their identities over to the characters, or the legends, if you will.
In Marvel, people with real lives sometimes have to put on their tights and be heroes. Perhaps the greatest example here is Peter Parker, who's girl troubles, family issues, bullying issues at school, social rejections, pure nerdliness, etc., were traditionally foregrounded just as much as the hero stuff.
Finally, it's worth saying that I'm painting in broad strokes here, and possibly talking about the "olden days of yore" for both companies. Clearly DC has opened up the personal lives of its heroes the last couple decades, while Marvel has moved some of its characters towards more of an iconic status. I think part of what made "Man of Steel" work this past summer is precisely that the main character is Clark more than he is Superman (of course, I seem to like this movie more than most of my friends, so what do I know? lol).