The first point we must consider in answering this is the respective ages of Wiesel and Levi at the time of the events they narrated. Wiesel was 14; Levi was 24. Elie Wiesel had the double misfortune of being too young to grasp a situation which even for an adult is incomprehensible. Primo Levi, as an adult, already was aware at the time he was arrested in Italy and placed on the convoy bound for the concentration camp that he and the others were en route to their deaths.
Another factor is that Wiesel was growing up in a religious environment in his home town of Sighet in Hungary. The first chapter of his book deals with his religious feelings, his studying the Torah, and his friendship with Moishe the Beadle, who in some sense represents the essence of the spiritual world to him. Yet Moishe is the one who is deported first, then escapes and returns to Sighet, where no one believes his account of the murders of the Jews he witnessed. The entire community has been sheltered from this knowledge until the mass deportation takes place and they are sent to Auschwitz. No one on the train grasps the prophecy of the seemingly mad Mrs. Schachter, who screams repeatedly that she sees a fire in the dark night outside the moving train.
Levi was basically a secular Jewish man who thus did not have to reconcile the cruelty and butchery with the faith in a benevolent God. Even so, his description of the arrival at the concentration camp is strikingly similar to Wiesel's. The entire scene is one of inconceivable madness, as if a mass psychosis has taken over those who wish to degrade others into a non-human state. The same horrors are described, largely in similar language, except that in Levi's account, there is nothing corresponding to Wiesel's visceral and terrified reaction upon seeing the crematorium and the piles of babies's bodies. For both men, the experience is one of shock and horror, but at least Levi was older, more aware and prepared for it, if anyone could be prepared for the atrocities.