What book(s) should I buy next?

I went to the book shop last night, and couldn’t decide between Aristotle’s Metaphysics and Nietzsche’s Thus Spoke Zarathustra. For those of you who have read these books, which do you think is better? Which is more relevant to the modern world? Are there any other books you would recommend, considering what I am already looking at?

I ended up giving up and buying the Volsunga Saga instead, so both are still on the list. I also have (but am yet to read) Aristotle’s The Politics, and have read Nietzsche’s The Antichrist, so I’m not sure if I will be doubling up on ideas and content.