How is that even a question? Of course I would save it. You say nothing good will happen, but that is obviously untrue.
If you see someone assaulting someone else, and you have the power to stop them, then any decent person should do it. If you see someone attampting a murder, and you have the power to stop it, then you should stop it. If you see that the world is dying, and you have the power to stop it, then you should stop it. Since you gave us the power to save it, then we should do so. Simple as that. It isn't even heroic to choose to help, that is just common decency. The heroic thing is to help despite all the personal hardships it entails to do so, but that is another discussion.
It is clear that the living conditions of this world are depressingly harsh, but that has no bearing on whether you should save it or not.
By that logic we could just destroy the poor African countries since they have harsher living conditions than us. The homeless people in our streets don't have the right to live or to be helped, because they suffer more hardship than us. Even we should die, because our comfort is nothing compared to the one enjoyed by millionaires.
No, we all deserve to be saved, because how comfortable your life is has no bearing on the value of your life.
Another point you raised was that you would delay the inevitable, but that is similarly irrelevant.
If you are infected by a deadly but easily curable disease, should the doctors refuse to cure you because you will die one day anyways? We should stop worrying about global warming, since the earth is going to be engulfed by the sun in four billion years anyways. I am sorry to say that we are all doomed, but that changes absolutely nothing to the value of our lives.
As for how to save it, whether it is by using your save option or becoming the master of the world, that depends on what becoming the master really means. If there is no drawback, then I will choose that, since it seems like a better version of your first option.
Before that, I would plan to restrict my powers as much as possible, because power corrupts. How I would do that would depend on what being the master of the world means, but if I let myself take control, this world would probably become a totalitarian nightmare very fast. I think the harm of me effectively stopping humanities philosophical and moral advancement would be greater than the good I would do by enforcing the moral standards of our current society (which I believe in).
p.s. I don't think a generic fantasy world could exist in reality. It does in books because the authors restrict them, but take a look at the variety present in nature and you will see there is no such thing as a generic world in real life.