Here's where I admit that while I definitely like Sam and Dean equally - they both bring important elements to the show - I can, on a base level, understand Dean more. I'm a person whose instinct, when I see a problem, whether it's a hurting friend or something more physical, is to fix it. I also want to protect everyone I like from ever being hurt, ever. I'm a twin, but I often, especially when we were younger, took the protective, older sibling role. (I remember grabbing the back of his shirt and keeping him from walking in front of cars. )
However, as much as I understand Dean on a really basic level, I like Sam, too. Sam is Dean's foil in a lot of ways. And neither would be who they are - yes, even now in season four - or make the decisions they make without one another's influence.
I've seen a particular phenomenon in the Supernatural fandom - among Sam fans, among Dean fans, even among fans who profess to love both. It happens in other fandoms with other characters as well, but people seem to be particularly vehement about it in Supernatural. They will demonise one brother in order to canonise the other. I recently had my inbox filled with LJ comments about this and I realised that it drives me INSANE.
It does not work.
Let's get something out of the way first.
Sam is selfish.
Dean is overly protective of his family, small children, and people (in that order). He comes in last.
Yes. Sam is selfish. He is. Hands down. Yes. Dean does have slightly messed up priorities.
Dean having messed up priorities does not mean that he does not have a sense of self. Sam being selfish doesn't mean he doesn't care about Dean.
These are called flaws. Good characters, like people, have them. Either exaggerating the flaws of the other character or ignoring the flaws of your favored character are only going to make you look like an ass (and drive me insane, because you should totally care what I think).
It's important, before you go off on how season four Sam is a selfish bastard addict or Dean is a weak-minded follower of orders wherever they come from, to look at how the characters function. Many books, shows, and movies that deal with other worlds or crazy science fiction or what-have-you have characters that function as windows into the new information. They allow the audience to understand and follow this new information that, presumably, everyone else already knows. After all, I don't need to explain the internet to my best friend, but if I were in a book from the nineteenth century, it might pay to have a period character asking me what it was. In Neil Gaiman's Neverwhere, Richard Mayhew functions as our "window." He is unfamiliar with London Below and asks the questions that will inform the audience. In Dr Who, we have the Doctor's companions. In Torchwood, we have Gwen, who is as new to Torchwood as we are. In the first three seasons, but especially in season one, Sam functions as our "window" into hunting. Nominally, he grew up in a hunting family, but, for the most part, he's led a fairly normal life. He has many of the same problems with it that we would. In the fourth season, Dean is our "window" into the world of demons and angels, struggling with many of the issues that we would.
From here, I'll be taking John Winchester's Journal and the Rising Son comics as an addendum to canon - canon unless they directly conflict with something on the show.
Taking the journal, the comics, and A Very Supernatural Christmas (3.08) into consideration, as well as some of Sam's commentary, especially through the first season, Sam and Dean had drastically different childhoods for all that they're brothers who grew up in the same household. (I am taking a fairly conservative view of their childhood here - the one that I see as having been directly depicted in the aforementioned sources.)
For everything else, Sam's childhood is very protected. Yes, we see in After School Special (4.13) that Sam deals with being a new kid and school bullies - and that he knows how to fight - but we know from A Very Supernatural Christmas (3.08) that he didn't even know the supernatural existed until he was eight. This allows Sam to see things in a black-and-white manner, something that, despite everything in Bloodlust (2.03), Dean doesn't do.
This is actually a great example of the divergence of points of view - and why saying Dean and Sam should be the same is a fatally flawed argument.
The example of the gun from the pilot is a good example of this: Sam is nine years old. He's afraid of the thing in his closet. John inspects the closet. There's nothing there. However, John knows, all too well, that evil things hide in closets. He is also very aware that evil things are after his youngest son, though Sam, a child, is unaware of this. John has to make a decision. He chooses that protection from evil is more important than innocence because it might be that his son's life is in the balance. He gives Sam a .45.
Sam finds this highly problematic, even as a mostly adult character in the pilot. Sam is used to a relatively (this being a key word) normal life. He goes to school. He gets good grades. His teachers talk to him about getting into college. He wants to escape his messed up home life where his father and brother disappear to go kill things no one else believes in and come back hurt. He can't understand why they hurt themselves over and over again for people who don't care. He feels rootless and unhappy and he is very much a teenager. Giving a .45 to a nine year old, to Sam, only shows how his family is messed up and has the wrong priorities. He wants to feel safe - away from people who collect weapons and are hurt often. For Sam, either you are safe or you are not safe. This is true even in season four. Giving a gun to a nine year old means you are not safe.
Dean does not see this as a problem. Dean remembers his mother being killed by a demon and his father totally losing it. Dean remembers what it was like to have a home and the pain of it all falling apart. He watched John get it back together, as much as John ever did, by focusing on the hunt. Dean learned to shoot when he was six. When he's eleven, he's a good enough shot and well enough versed in hunting, that even the over-protective John is willing to leave Sam with Dean while a Shtriga is on the loose. This is Dean's life. When he's twelve, Dean kills a man, another hunter, to protect Sam.
To Dean, giving a nine year old a .45 is a protective measure. It makes sense. Dean has seen what happens when you don't take protective measures. Dean's seen the death and pain. Dean doesn't want to be "normal" because he was, once. Dean knows, intimately, the pain of what happens when that's torn away. For Dean, there's no "safe" and "not safe." There's "protecting yourself and those you love from things that will hurt you" and "being a lazy ass."
For Dean, his mother was taken from him by these supernatural things. He has motivation to stay in this life. He also has motivation (motivation Sam doesn't have) to need to keep his family together. He's seen what happens when things fall apart and it isn't pretty.
Sam sees things in black and white, which was a problem in season one and is a problem now. Currently - his end result is good so the means are justified (ie because it's not 100% pure evil, it must be 100% pure good). In season one, it meant that he could only find John, other things were distractions. For all that he's the human with demon blood, Sam is blind to gray areas - and blind to compromise. (I admit, I'm one of the fans who has serious, serious problems with Sam drinking demon blood. I think that it's darker than anything Dean, John, and Mary did because it's much less spontaneous and he's very much choosing to use it.)
Dean is the protector which, like Sam, is his strength and his downfall. He's known more than Sam for most of Sam's life. He knew about hunting when he was just a little kid. (If you take the journal and comics into canon, he's known since John did.) He knew that Sam might go darkside before Sam did. But he also doesn't know when to quit. It's one of the major problems of season four Dean - he's understandably broken by Hell but he doesn't know how to step down. (When you mix that with Sam's black-and-white world view of "Either he can do or he can't," I think Dean got more fucked up than necessary.)
Sam's black-and-white thinking is part of what makes him a strong hunter and a strong character. He can make definitive decisions. In Scarecrow (1.11), he can make the decision to go after John because finding his father, avenging his girlfriend, is definitely more important that saving strangers. Dean's protective nature keeps him from this. He feels the need to protect. In Scarecrow, this means that saving strangers takes a higher priority than going on a cross country wild goose chase. Dean is more likely to be torn up over a decision, especially if he has to choose between people to save. Sam sees the end result and if it's good, the whole issue is good. If the result is bad, then it's all bad.
They're both got their problems (huge, honking problems). They've both got their strengths (huge, honking strengths). But really saying that one problem is a virtue and the other is damning just doesn't work.