"Our Man in Havana" has a lighter touch, and is very funny in places; while "The Quiet American" was incredibly prophetic, given it was written in 1956. Here's a New York Times review written at the time, scoffing at Greene's warnings (warning - review contains spoilers):
Graham Greene's new book is quite different from anything he has written before. It is a political novel -- or parable -- about the war in Indochina, employing its characters less as individuals than as representatives of their nations or political factions. Easily, with long-practiced and even astonishing skill, speaking with the voice of a British reporter who is forced, despite himself, toward political action and commitment, Greene tells a complex but compelling story of intrigue and counter-intrigue, bombing and murder. Into it is mixed the rivalry of two white men for a Vietnamese girl. These elements are all subordinate to the political thesis which they dramatize and which is stated baldly and explicitly throughout the book.
As the title suggests, America is the principal concern. The thesis is quite simply that America is a crassly materialistic and "innocent" nation with no understanding of other peoples. When her representatives intervene in other countries' affairs it causes only suffering. America should leave Asians to work out their own destinies, even when this means the victory of communism.
...
If much of the description of Indochina at war is written with Greene's great technical skill and imagination, his caricatures of American types are often as crude and trite as those of Jean Paul Sartre. He is not ashamed as an artist to content himself with the picture of America made so familiar by French neutralism; the picture of a civilization composed exclusively of chewing gum, napalm bombs, deodorants, Congressional witch-hunts, celery wrapped in cellophane, and a naive belief in one's own superior virtue.
http://www.nytimes.com/books/00/02/20/specials/greene-quiet.html?_r=1&oref=slogin