I've been thinking about this the past few days. Christians think that the universe as it is is perfect, because it was created by God. Atheists, on the other hand, think that the universe is just one big cosmic accident. Therefore, there is always room for improving things. The atheist knows the universe is not perfect, and wants to do all they can to improve themselves and their environment. Doesn't it seem like if one thinks the universe is perfect that they would be more complacent about things that are obviously wrong, since it's all part of a perfect plan anyway?
From what I've read, this actually comes close to the beliefs of Mother Theresa. She didn't actually help sick people so much, she just gave them a comfortable place to rest while they were waiting to die and go meet God.
As a corollary, the atheist would do all they can to stave off death, while the Christian is more accepting of death because they believe there is an afterlife. Now who is more moral? If I had a choice I would definitely prefer that my doctor was an atheist. If I had a serious medical problem I wouldn't want my doctor saying something like "It's in the hands of God now." I would want them to try anything to keep me alive.
Which is more moral? I think the question you really want to ask is, "Which is more
effectively moral?" Which is to say, "Which of these two opposing mindsets has done, does, and will do more for the human race?" (As opposed to "Which of these two mindsets
wants to do more?") In that case, what the two sides think of the nature of the world and universe is rather a moot point. It's what they get up and do with that mindset that really matters, and that's the really tricky part--some Christians are very go-to human-helpers, others are more passive, and others might just sit back and leave it all to their unseen God. Atheists are just as varied, though the latter of them don't leave it to an unseen God... they just kind of leave it, period, which I'd take to be less internally moral (but that, as I said, doesn't seem to be the thing you were really driving at).
Now, oftentimes it seems to me that Christians are generally more motivated than atheists to get up and do something, probably because their beliefs are so firmly rooted in the concept of love (love for your friend, love for your enemy, love for your neighbor... Good Samaritans all around). And I admit that I've never encounted a charity organization that was atheist at heart. But there are a number of factors to consider--for one, maybe the Christians and other religious folk simply outnumber the atheists, so it looks like the religious folk are doing more on an individual basis when it's just that atheists haven't had the chance to really show their moral metal; also remember that a charity begun by an atheist might not want to parade the fact that it's founded on atheistic morals, either because it thinks that might repel potential members or because it simply sees no need to parade the fact that it's atheist--in which case an initially atheist charity may over time come to include many members of varying religious beliefs, all of whom participate for their own different motivations to be charitable.
Beyond all of that, I feel my present informational status is insufficient to confidently add anything to this discussion, so I'll end this here with the simple statement that I highly doubt, at this point, that either of the two mindsets described by Ragnar is more or less moral than the other.