If Ralph is more distraught by Piggy's death than by Simon's, it is because Piggy was his first real friend on the island. Perhaps more importantly, it is alluded to that Simon's death was an accident while Piggy's death clearly was not. They boys mistake Simon for the beast and kill him. When Roger kills Piggy, it is not an accident. Piggy's death signals the end of order and civilized society. So, for Ralph, Piggy's death is a bit harder to take because he felt a closer kinship with Piggy, Piggy's death was more unjustifiable and because Piggy's death symbolized the end of innocence.
However, at the end of the novel, it seems apparent that Ralph is crying for Simon, Piggy, and the end of their innocence:
But the island was scorched up like dead wood--Simon was dead--and Jack had . . . The tears began to flow and sobs shook him.
His voice rose under the black smoke before the burning wreckage of the island; and infected by that emotion, the other little boys began to shake and sob too. And in the middle of them, with filthy body, matted hair, and unwiped nose, Ralph wept for the end of innocence, the darkness of man's heart, and the fall through the air of the true, wise friend called Piggy.
Ralph begins to cry, thinking of Simon. But Ralph's sobbing does become more intense in considering the loss of innocence and finally becomes most intense as he's thinking of Piggy. Simon and Piggy were innocent and Ralph weeps for them both. Ralph might feel more of a loss with Piggy's death because Piggy was true and loyal to the end, never giving up on Ralph, never giving up on peace; Piggy continued to believe that the boys could restore order, that the conch could be used to make their lives peaceful and innocent again. Piggy's death signaled the end of that possibility; therefore, to Ralph, Piggy's death was more symbolic of the boys' loss of innocence than Simon's.