I think a good comparison point to stress between the two men is that both men work tirelessly to help the bomb victims. Tanimoto is in a location that affords him the opportunity to get away from Hiroshima, yet he runs toward the destruction in order to help in any way that he can.
He had thought of his wife and baby, his church, his home, his parishioners, all of them down in that awful murk. Once more he began to run in fear—toward the city.
We see a similar determination to help from Sasaki. He winds up being uninjured from the blast, and that allows him to be in a position to help victim after victim in the hospital.
By three o'clock the next morning, after nineteen straight hours of his gruesome work, Dr. Sasaki was incapable of dressing another wound.
Regarding a difference, I think that it is best to examine how the bomb affects their later lives. Sasaki doesn't believe that the bomb is the most defining moment in his life or career. Instead, his lung problem and lung removal wind up being more life altering.
In later years, Dr. Sasaki came to think of that experience as the most important of his life—more important than the bombing. Haunted by the loneliness he had felt when he thought he was dying, he now did his best to move closer to his wife and children—two sons and two daughters.
That is quite different from Tanimoto. The bomb and its effects so deeply impact him that he makes it a life goal to promote peace and speak out against the use of such weaponry.
On the sea voyage, an ambitious idea grew in his mind. He would spend his life working for peace. He was becoming convinced that the collective memory of the hibakusha would be a potent force for peace in the world, and that there ought to be in Hiroshima a center where the experience of the bombing could become the focus of international studies of means to assure that atomic weapons would never be used again.
A possible thesis to guide this compare and contrast essay could be something similar to the following thesis statement: "Although Tanimoto and Sasaki had similar experiences in helping the bomb victims, each man had a completely different takeaway from the nuclear disaster."