Choose the subject you think you would 1: enjoy the most or 2: do well at. Considering you have already completed business management once, I personally would recommend you to choose it again to deepen your grasp of the content and potentially attain a higher study score. However, you must be willing to put in the same amount of effort as your other subjects and don't think you can slack off because you've already completed it once. Focus on learning the ins and outs of the subject so that you know it completely. On the other hand, if you enjoy further mathematics more, and would be more interested in further, then I would recommend choosing that as you will put more time and attention in the subject. Catching up on the 1/2 content would definitely not be an easy task, but if you have a genuine interest in further then doing it throughout the year will be much more rewarding. In the end, it's solely up to your understanding of your own capabilities and whether you think you can take up a year 12 subject without doing year 11. If you're a lazy person however, I would recommend against it.