A debate between Christopher Hitchens and Alister McGrath has been posted on "The Official Richard Dawkins Website". In my opinion, the typical person who enters the debate "on the fence" will most likely leave siding with Hitchens.
This is not due only to Hitchens's polemical acumen (or at all to McGrath's lack of talent -- he's certainly not lacking for that). Rather, I think that any debate set up as this one seems to have been will always favor the atheist (not to say the atheist will always get the better of the theist).
On the other hand, what can the religious believer say to the atheist? It seems to me that what he has to do is either find a contradiction in non-belief in God (perhaps by having a powerful ontological or two-stage cosmological argument) or a contradiction between atheism and some other belief the atheist is likely to have. But if all I know about someone is that he is an atheist, then I will have a great deal of difficulty with the second tack, because I'm not likely to know what other beliefs the atheist has, say about morality, qualia, abstract objects, etc.
An analogy can be given. Imagine a debate between a moral nihilist and a Kantian (or a utilitarian, virtue theorist, etc.). The moral nihilist will have a grand old time bringing up perennial objections to the Kantian, but the best the Kantian can do is point out how the nihilist goes against common sense. (Note that this goes for a debate even between an atheist and a non-religious theist.)
Second, and much related to the first difficulty the theist has, quickly articulating a powerful objection to a position is often, perhaps usually, much easier to do than to respond to it. However, although it is easier to offer an objection to a position than it is to respond to it, it does not follow that the position objected to is weaker. For instance, one can very quickly and easily present the dilemma of determinism (which was, when I first encountered it, quite unsettling); but offering a compatibilist or libertarian response to it takes a lot more nuance and thought. If someone were watching a debate between a compatibilist/libertarian and a hard determinist, though, and didn't have a dog in the fight (if that were possible), then what he would see if someone constantly qualifying, elaborating, stuttering here and there, and saying things that were hard to understand (this would be the believer in free will), whereas the skeptic about free will would have a clear, persistant line of reasoning that he could repeatedly rephrase in florid language (should he, like Hitchens, have that talent).
My third point is a difficulty that is perhaps particular to the Christian debater. The atheist, should he want to, can impugn Christianity as "poison", "maniacal", "immature", "deranged", "a death cult", and so on, but if the Christian were to respond in kind, he would look hypocritical. On the other hand, if he doesn't (as he shouldn't), then he might look as though he doesn't have confidence in his beliefs. In fairness, though, Hitchens's stridency might also turn off undecideds.
One last thing, which doesn't have anything to do with the nature of public debate between an atheist and an adherent of a monotheistic religion, but rather with this particular debate between Hitchens and McGrath. Hitchens was questioned about what makes it the case that we ought to carry out our moral obligations, and his response was that we have a natural genetic predisposition to do so. It goes without saying, at least here, I should think, that this answer, which Hitchens appeared to think sufficed, is a non-starter.