Well..there is an expectation in extension. In advanced you're taken care of and all the answers are fed to you. I personally chose extension 1 because I wanted to be doing something 'worthy' rather than 'photography' or something. I also wanted to expose myself to broader range of texts. i kind of see ext and adv as one subject - it's english after all. you may find that doing extension will help improve your marks in advanced because of this. Just do it if you have nothing else that you're really interested in. im dropping it next year anyways, so no pressure for me. it's meh if i dont do so well haha. so long as adv is going strong.
happy studying.