Cutting to the chase here on the blog, I believe that a key point in favor of theism is the issue of free will, or volition. In the atheistic paradigm, man is composed solely of physical stuff; there is no spiritual component. This means he is driven by a combination of genetics and environment, determinism is the rule, and free will cannot exist. The theistic system allows for free will, and the Christian system positively affirms it.
Ironically, this rational point reveals the relative unimportance of rational argumentation. Sound reason may lead to faith, but if we have free will, each individual has the ability to choose whether or not to believe, whether for rational reasons or otherwise.
The opposite also applies. If we lack free will, rational disputation is pointless. The determinist says that everyone behaves in a determined way. He says to the Christian, "You believe because of your genetics, upbringing, and environment." The Christian replies, "You choose to disbelieve." It's a rational stalemate.
My earlier point still applies (see "Pseudo-atheism" below). Within the system of materialism and determinism, evangelism is absolutely pointless. Within the free will system, rational disputation may not sway many minds, but, together with other means of evangelistic outreach, it does at least make some sense.
I, for my part, have chosen to believe that I have the freedom to choose. Why? Because it's more reasonable.