It depends on your goal. But on the QPdfWriter side, you have one less module to include and have more control about what you can put it there. But your users cannot send it to a printer.
@ademmler Like @SGaist said. If your solemn goal is to save as pdf, go with the writer. If, on the other hand, actual direct printouts should be possible - QPrinter is your friend for the sake of the flexibility.