Imagine a parallel world which is exactly like ours, except Mars has an Earth-like atmosphere of nitrogen, oxygen, and argon. There's no more (or less) life on Mars than there is in our world, but it could much more easily support transplanted life from Earth.
Which is the better world?
My first inclination is to say, "The world in which Mars has an Earth-like atmosphere is the better world!" After all, the Earth keeps getting more crowded. We could benefit from a nearby planet with breathable air to colonize. But wait! The question wasn't: "Which world is better for humans?" If the two questions were identical, it wouldn't be possible to say one human-less world is better than any other human-less world.
Two other questions which aren't identical to the original:
"Which world is better for conscious beings?" (Including dogs, aliens, etc.)
"Which world is better for living beings?" (Including plants, bacteria, etc.)
Or variations of this sort:
"Which world is better for allowing complex structures?"
"Which world is better for maximizing happiness?"
Even when we can answer these more specific questions, the answers may conflict. A better world for humans might be a worse world for wolves, buffaloes, and whales. A better world for maximizing happiness might be a worse world for maximizing social equality. Can the original question be answered without changing it into something more specific? How could a world be simply better rather than better for some things or ideas?
I've become suspicious of this "simply better" idea. It might be a mistake which comes from so often hearing "good" and "better" without an explicit qualification. We usually communicate accurately anyway because the intended meaning is clear from context. But when the meaning isn't clear, I think it's most appropriate to ask "better how?" If that can't be answered, I would assume the other person is merely expressing a preference rather than making a claim.
So my answer to "Which is the better world?" is "Better how?"