The gospel(s) and the creation narrative are completely different genres of literature. Neither are what I would call "allegory", although both contain some allegories. Genesis is what I would call "myth", although this term is used differently in popular English than it is in literary studies, so we need to be careful in delineating that Genesis is myth in a certain sense and not otherwise (the Barthian sense?).
The book of the Bible that is most clearly allegory is the Book of Revelation. It is apocalyptic allegory. One of the largest problems in contemporary American religion (Left Behind, et. al.) is due to a mis-reading of Revelation as "future" history rather than allegory and gospel.
Why are the gospels different? Well, they are eye-witness accounts, or literary performances of collected eye-witness accounts. It would have been impossible for Moses (the "author" of Genesis) to be present at the creation of all things, not to mention Adam and Eve, nor could anyone that we might call an eye-witness have been there- so the Genesis myth is by its very nature going to be different from the historical account we have of Christ's life, death, and resurrection.
I believe in and confess the resurrection of Christ because I have heard it from people who have heard it who themselves heard it from... on down through a chain to the original eye-witnesses. See Kierkegaard's Philosophical Fragments for an interesting discussion of this issue of believers at first and second hand.
**We might also note that this issue of eye-witnesses and the passing on of the story from generation to generation is of the very essence/reality of the church. The church is that community of people who have heard the good news of Christ's resurrection (and just so participate in it), and are passing it on to future generations.**
It has been popular in some circles (beginning with Feuerbach?) to read the resurrection as allegorical wish-fulfillment, as in- because Jesus now lives on "in our hearts" after his death, he has been "resurrected". This collective belief in Christ living on then emerges, supposedly, as an actual claim by the community to his resurrection.
But this seems patently false. The first eye-witnesses began reporting immediately these things. 1) An empty tomb. 2) Appearances of the risen Christ. 3) An Ascension. 4) Multiple witnesses to the same resurrection.
I think it takes considerable courage and faith to believe that a group could collectively cleverly devise such a "collective unconscious" and then spread it world wide. Nevertheless, to believe thus requires a stubborn self-actualized faith of almost similar magnitude to actually simply confessing the resurrection of Christ, and the promise of our participation with him in it. We can all create our own religion if we wish. I feel no need to do so.
This is finally why the resurrection and the creation narrative are of completely different orders for me. The first is a narrative of God's relation to the old creation, how it was brought about. Certainly, this is important. But it is not of salvific importance to get it "right" or to believe it in certain ways as historical. On the other hand, that Christ is raised from the dead, historically and really, is very faith of very faith, the historical reality that grounds the church as that community that proclaims already the New Creation, Jesus Christ and Him crucified and risen.