These two authors share much in common: both have relaxed styles, a feeling of spontaneous invention at novel lengths, a penchant for humor derived by putting humans in awkward and bizarre scenarios.
They do differ in literary approach, however. While Rucker's characters tend to feel more fleshed out, Sheckley's emphasis on examining the human condition spurs breadth or depth of thought. Rucker also examines the human condition but as a consequence of examining some mathematical or physical anomaly in much the same manner as Edwin Abbott or Hal Clement, who take concepts and push them to extremes. A child-like wonder suffuses Rucker's approach, seen for instance in "Inertia" where Rucker asks if there might be a difference in masses from gravity to that determined by force. In Masters of Space and Time, Rucker inquires as to how one might travel forwards or backwards in time: by shrinking or enlarging, depending on the direction one were to travel.
Sheckley, meanwhile, might try to separate the best and worst of human nature in a manner not unlike Stevenson's "Dr. Jeckyl and Mr. Hyde" presents two sides of one man. Sheckley's Status Civilization, however, operates on a societal level, sifting and isolating the undesirables to one planet and saving Earth for the good people. Here Sheckley devotes his time to examining questions of "status" and its permutations of meaning between these two societies in addition to questions of what "civilization" is.