Neither one is more believable than the other? That doesn't make sense. Atheism has nothing in that requires belief. It just means that you don't believe in deities. The reasons given for individuals being an atheist can be analysed, but that isn't an intrinsic part of Atheism itself.
Christianity, however, is specific. It states that there is a God who created the universe, and he did it a certain way, in a certain order, within a certain timeframe. It tells you the names of the first human beings created. It tells you all of God's interaction with humanity in chronological order. It gives you ten rules to follow life by, and if you don't, you won't join God in the kingdom of heaven. It tells you that a man called Jesus existed in a known, recorded part of history, and that he performed several miracles until he was crucified, and then he came back from the dead.
Christianity is, by several orders of magnitude, far less believable than Atheism. Atheism doesn't ask you to believe anything.