I've said before that the purpose of the gospels, or the Scripture in general, is not to teach us exactly what happened in history by relating every little historical detail as though we were right there watching it on TV. The primary purpose is to relate the work of God and the theology/ethics He wishes to teach His children as a part of His work in the world and in us. So Scriptural details should be read more like one assesses the details of a painting. They may simply exist to connect other parts of the painting to one another, relate a more realist exactitude of the detail, or be altered in order to convey a concept through the detail that contributes to the message of the artist.
Having said that, I am also very skeptical toward anyone claiming that one report of an historical event must be wrong because it contradicts another report if taken at face value. Anyone who has ever really thought about this idea at length should immediately dismiss this as the default position to take within one's methodology of historiography. Truth is stranger than fiction. What should, in reality, be the default position, and was before postmodern, "everybody's out to deceive me in order to oppress me," historical deconstructionism came along, is trusting that either the details in the reporting sources are likely linked to the event in some way, or the author has altered them for some reason (whether that be to deceive or craft a more literary friendly presentation of the reported event). But imagine putting the pieces together from just four reports of what occurred on 9/11. Most reports only mention the two planes that went into the towers in New York. That's not because the reporters are unaware of the other planes. That's because the two planes are the ones we saw on TV, they are the ones that hit the biggest target with the most destructive consequence, etc. In short, they, as the two with which we "interacted" the most represent the other planes within those reports. But if one were to attempt to reconstruct the history from those reports, as many Gospel critics do, one would conclude that either there were two planes or there were four. What, then, would occur if only the three that hit buildings were mentioned? Then we would have a three way contradiction. Yet, those of us who were here and know the facts, know that there is no contradiction at all.
The same goes for the buildings. How many buildings went down on 9/11 as a result of the bombardment? Most reports, again, only mention the two towers. Yet, other reports mention the Pentagon, and still others (although not many) mention the 7th tower along with the two others, and even less mention the Greek Orthodox Church that was destroyed. There were also many buildings that had to be demolished because they were damaged so badly. Yet, if you read some of these variant accounts two thousand years from now, you'd think that many of them are erroneous. That's why true historiography, a historiography that attempts to reconstruct what actually occurred, cannot approach documents with skepticism but with a certain amount of trust instead. If one does not wish to trust those documents, his reconstruction must now be made upon even less documentation. Thus, it is less likely that he will come to the truth of the matter beyond hitting the numbers via an historical roulette of speculation.
This brings me to the Gospels. Much is made of the variations between the details, but I want to point out something very important. Noting the above about the reports concerning the details (and these are big details) that surround the events on 9/11, one ought to approach the Gospels in a manner that understands that truth is stranger than fiction, that variation can, but rarely does, mean contradiction in historical reports, and that the task of the historian is not simply to easily dismiss variation as contradiction, but attempt to reconstruct the event with what he has (again, I'm not saying the Gospel reports exist so that we can reconstruct the details of the event, but if one is going to do that, he needs to approach them in a way that will yield more than his pure speculation would).
So in Luke's and John's Gospels, there are two angels mentioned. In Matthew and Mark, there is one that is mentioned. In the Synoptics, Mary is accompanied by other women, but in John she appears to be alone. I say, "appears to be," because I'm going to use this now as an example of why critics are not doing good historiography here. Again, it's possible that there is contradiction between historical accounts, but it's clear in the case of the Gospels that this is not the issue.
For one thing, Luke knows at least Mark, if not also Matthew and certainly the sources (likely oral tradition) that are common to the Synoptics (often referred to as Q). So Luke isn't mistaken. He knows that other reports say that there was an angel. He, however, reports two angels. We can debate why that is, how it fits into Luke's narrative and theology, the nature of his Roman audience and their beliefs, or whether it is just his wanting to bring out a detailed report, etc.; but we cannot say he is ignorant of the facts and therefore in error. His decision to include two angels instead of one is purposeful, much like anyone wanting to include all of the buildings (or some of the others) that went down due to the events on 9/11 rather than just two. The angel in Mark and Matthew can be seen as merely representing the other, as it is only important to mention that the declaration that Jesus is risen was angelic. One does not need to know how many angels were actually there.
But someone may disagree. Let me then put a nail in the coffin. In John's Gospel only Mary the Magdalene is mentioned. Hence, John must contradict the other Gospels that report that many women were with her. I mean, the whole Johannine narrative seems pretty clear that she is the only one speaking and present, right? Wrong. It seems this way if you get the information only from the flow of the narrative. John doesn't bother mentioning the other women, because Mary's interaction is most important to him. Mary is also listed as the first person within the group in the other gospels, because she is most prominent (well known to the Gospel audiences as representative of the group of women who followed Jesus). Hence, just like we represent all the planes with two planes, and all the buildings with two towers in our reports, details like these in historical reporting often stand in as representative of other details, simply because one has to do this if he or she is going to make an historical report (you can't write a summary account of events without doing this).
Now, again, some may disagree; but here is the kicker. In John's account, Mary mentions that others were with her. But if you don't read it carefully, you'll miss it. Notice in 20:2, it says, "So she went running to Simon Peter and the other disciple whom Jesus loved and told them, 'They have taken the Lord from the tomb, and we don’t know where they have put Him!'" Did you catch that? First, notice that it says that she went running to Simon Peter. Again, there is no mention of the others, as she is representative of the others. But how do we know there are others here? Look at what she said. We do not know where they have put Him!" Who's the "we"? Not the angels. They know where Christ is. It isn't the disciples. She's reporting to them that "we" don't know where they have laid Him. So the only possibility is that the "we" refers to the group she is with. Yet. throughout John's account, the way it is presented, you would never know that they are there otherwise.
You see, I would be much more cautious if I were you in reading these texts in order to look for what contradicts what, as skeptics often do. Of course, most skeptics are just looking for contradictions, and this is easy fodder for the sloppy scholar. But in reality, most reports, ancient or contemporary, will seem like they are made up if we have multiple accounts of the same event that vary in their details. The problem with that is that any event is multifaceted and reported from a particular perspective of various individuals. That's true, even when one is not constructing a theological-literary work of art in order to present that history in the way the Gospels do.
Instead, we ought to approach them in a more positive light, looking to see how these might fit together, or at least, why the details are varied in a particular Gospel. Contradiction is the easy path. It's the most noble in academia because it supposedly evidences that one is not blinded by faith in his methodology of inquiry. Such faithless objectivity is a modern fiction within itself, but the point I'm making is that even in doing purely "secular" historiography, the best chance one has in reconstructing an historical event is to take all of the accounts and attempt to understand why the variation exists and how it can possibly exist together, since, apart from deliberate lies (which I believe are not as likely within the communities within which the Gospels are written), variations most often arise from something rooted in the event. The task of the historian is to find out how they fit into the actual event or why they look the way they do in light of that event. Either way, until all other options have been exhausted, "contradiction" is a lazy and inappropriate word for such a task.