In "The Giver" by Lois Lowry, the society lives in a controlled environment where emotions and memories of suffering have been completely erased. The people in this community don't know about the pain and hardships that exist outside their perfect world. For example, they can't understand concepts like war, hunger, or sadness because they have never experienced them. Because of this lack of knowledge, they believe they are happy, but their happiness is built on ignorance.
On the other hand, in "The Ones Who Walk Away from Omelas" by Ursula K. Le Guin, the citizens of Omelas are fully aware of the suffering that exists in their society. They know that their happiness and prosperity depend on the misery of one innocent child who is kept in a dark, filthy room. Everyone in Omelas knows about this child's suffering, and they must choose whether to accept it or walk away from the happiness of their city. This awareness creates a deeper moral conflict in Omelas, as the people must confront the reality of their happiness being built on someone else's pain.
In summary, the key difference is that in "The Giver," the people don't know about suffering at all, which means they cannot question their happiness, while in "Omelas," the people are fully aware of the suffering that allows their happiness to exist, leading them to make difficult choices.