In a compare/contrast essay addressing The Stranger and Slaughterhouse-Five one comparable point common to both is the theme of free will. Both novels address free will and its importance to life but they address it from contrasting perspectives. In The Stranger, Meursault is preoccupied with following and assessing his condition of freeness, a totality of freeness that is unsettling to observers and may be mistaken by other characters and readers for indifference. In Slaughterhouse-Five, the insistence is that free will doesn't exist in the same that time as a progression doesn't exist; all time is locked into the present with the past, present and future existing as a simultaneous eternity: "always have existed, always will exist." In discussing the theme of free will in both books, you have a point of comparison and a point of contrast.
Another point that is comparable is that both books were outgrowths of the authors' experiences in World War II. As in the above discussion of theme, this common feature also has an element of contrast in that The Stranger was written and published in 1942 during the time of World War II following France's surrender to Germany, whereas Slaughterhouse-Five was written and published in 1969 as a recollection of past experience.
A related point is the authors' philosophical perspectives. Camus (The Stranger) believed in an absurd and meaningless universe--an existential universe--in which the individual is nonetheless required to uphold the traditions of human values despite the perceived meaninglessness and absurdity. Vonnegut (Slaughterhouse-Five) similarly believed that even in a seemingly random and meaningless indifferent universe--a seemingly existential universe--the individual is required to uphold the human values of kindness and decency in behavior toward and treatment of one another.
One point of comparison between The Stranger and Slaughterhouse-Five is the element of absurdity. The protagonists of both novels Meursault and Billy Pilgrim watch their lives unravel as the status quo of their existences is challenged. The death of Meursault's mother sets his life on a course of change, and he cannot deal with the shifts in his life. He eventually is charged because he does not appear to have an appropriate sense of grief and remorse over his mother's death, yet Meursault is so overwhelmed by the expectation of his grief that he cannot really feel anything. Billy Pilgrim becomes "unstuck in time" and enters a journey of shifting time and reality as he travels through the war, his own past and future, and Tralfamadore. So, both novels explore the nature of the absurd as Meursault's and Billy's lives begin to unravel.