There are different ways to examine the themes and motifs of these novels to compare and contrast the relative optimism and pessimism of the writings.
Let us start with The Fifth Head of Cerberus, which provides a biographical insight of the protagonist's life on an alien world. The protagonist, “Number Five” or Wolfe, goes through a number of ordeals which included forced experimentation, torture and imprisonment. The protagonist also murdered his own father who was responsible for much of his initial suffering (through torture and experimentation).
The novel itself is actually three tales in one with the intertwined stories converging on each other to reveal the eventual plot line having no clear salvation for the protagonist. The second story in the series of three is "John Sandwalker" and focuses on deceit, murder and the eventual usurpation by humans of the planet from the native aborigines. The third story is “Dr. John V. Marsch” and deals with his imprisonment and torture as well as his coming to terms with the realities of his identity and heritage.
The Old Man’s War, which is also relatively pessimistic, deals with themes of constant war, conflict over resources, human ethical issues and coping with loss. In Old Man’s War, humanity is in a life or death struggle with a myriad of aggressive alien races. To combat this threat they use genetically enhanced senior citizens who are drafted as the military vanguard against the alien menace. Throughout the novel the protagonist, John Perry, deals with the loss of loved ones and with having to cope with the stress of constant warfare and battle.
In contrast to The Fifth Head of Cerberus and Old Man’s War, Nova has a more upbeat, less pessimistic, theme. Although there is also struggle and conflict in this novel there is at least hope and salvation for the protagonist, Lorq Von Ray, and the other main moral characters. The theme is a generally rosier one, with the protagonist getting more powerful as the story progresses and coming closer to achieving his goal. It also portrays a future society that, although culturally stagnant, is more peaceful and at ease with itself than the societies in the other two novels.
Out of all the novels, Nova seems like a more plausible space faring future because it is the least speculative of the three works. Old Man’s War and The Fifth Head of Cerberus deal with the results of contact and conflict with extraterrestrial species. This is a highly speculative theme since we have yet had or confirmed contact with an extraterrestrial race, so we have no basis from which to measure the possible outcomes of such contact. On the other hand Nova deals with the internal struggles that occur within the human species, issues like race, social class, liberal versus conservative, culture, and progress. These are themes and motifs that can be examined, forecast and extrapolated based upon an historical perspective.