Both stories in Life of Pi are equally believable because they are both presented as equally plausible. Pi himself says that both stories are true, and that it is up to the listener to decide which one they prefer. However, the story with the animals is more interesting and captivating, and it is also the story that Pi prefers.
Mr. Okamoto reports the story with the animals because it is the one that he was told by Pi, and it is the one that he believes to be true. Although he is skeptical at first, he comes to accept that the story is plausible and that Pi truly experienced what he says he did. Ultimately, the story with the animals is more compelling because it speaks to the power of imagination and storytelling, and it allows us to see the world in a new and different way.