How do the novels Things Fall Apart and Heart of Darkness differ in their portrayal of colonialism?

One of the main differences in the books's respective portrayals of colonialism lies in their use of perspective. Heart of Darkness is told from the standpoint of a colonialist, albeit a colonialist who comes to question the very nature of colonial rule, whereas Things Fall Apart is a tale told by the colonized, those on the receiving end of the colonial enterprise.
As the latter perspective comes from the inside out, as it were, it gives us a much better idea of the damage that colonialism does to indigenous societies. Conrad may depict the sheer brutality of colonial rule with suitably harrowing realism, but the brutalized natives that Marlow encounters are never more than objects. The villagers of Umuofia, by contrast, are living, breathing subjects—inheritors of a rich, ancient civilization that is gradually being corroded by the domination and control of Western colonial rule.
Apart from anything else, this insider perspective enhances our understanding of what colonialism involves, helping us to see black Africans in a new light: not as the Other, as in Hearts of Darkness, but as subjects in their own right.

The representations of colonialism in Things Fall Apart and Heart of Darkness differ in a major way. In Heart of Darkness, Joseph Conrad depicts colonialism as a violent force deconstructing a society of savages and barbarians. In contrast, in Things Fall Apart, Chinua Achebe depicts colonialism as a violent force deconstructing a sophisticated and highly developed culture. As such, though both novels display the savage nature of colonial enterprises, Conrad makes the assumption that cultures subjected to colonialism are entirely primitive, whereas Achebe recognizes the nuanced, complicated, and culturally advanced natures of societies dominated by colonialism.
This distinction is critical. While Conrad's critique of colonialism is very important, it's undeniable that he depicts African cultures in a racist manner: native Africans are represented as primitive and less-developed than Europeans, and the latter are driven mad by the dark and "primitive" cultures of Africa. Achebe, however, depicts his native Igbo individuals as normal human beings: they can be virtuous and noble, they exist in a complicated social hierarchy, and they also make entirely human mistakes. Thus, Achebe's depiction of colonialism is ultimately harder to bear. While Conrad shows colonial powers destroying apparently "savage" cultures, Achebe shows colonialism destroying sophisticated and cultured human beings. In this way, Achebe truly gets to the heart of colonial injustice.