Mike.
1. He doesn't have the attachment to family that Vito does. Vito backs down from a war to protect Michael and Fredo after he sees what happens to Sonny. Michael kills Fredo because he is a weakspot.
Vito didn't back down to protect Mike and Fredo. He step up a meeting for peace talks because he knew that Tattaglia and Sollozzo were being backed by one of the other bosses and he needed to figure out which one. In the scene after the meeting when he's in the car with Tom he tells him that he knew someone was backing them but he didn't know until then that it was Barzini. He wanted Mike back because he'd be useful once they made the move to go back to war but Fredo stayed in the bushes where he belonged because he would only be in the way.