If Voldemort had marked Neville Longbottom as his equal, instead of Harry Potter, would Neville have survived the Killing Curse as Harry did? Neville and Harry are both inherently brave; however, their personalities are quite opposite. What would have happened if Voldemort had marked the Longbottoms for death instead of the Potters? How would history have been different? Here are a couple relevant passages from Order of the Phoenix:
Sybill Trelawney's prophecy:
"The one with the power to vanquish the Dark Lord approaches. Born to those who have thrice defied him, born as the seventh month dies. And the Dark Lord will mark him as his equal, but he will have power the Dark Lord knows not. And either must die at the hand of the other, for neither can live while the other survives. The one with the power to vanquish the Dark Lord will be born as the seventh month dies . . ."
Order of the Phoenix - Page 841 - US Hardcover
After relating the prophecy to Harry, Dumbledore says:
"You are forgetting the next part of the prophecy, the final identifying feature of the boy who could vanquish Voldemort. . . . Voldemort himself would 'mark him as his equal.' And so he did, Harry. He chose you, not Neville. He gave you the scar that has proved both blessing and curse."
Order of the Phoenix - Page 842 - US Hardcover