I think it really depends what you are more interested in. Personally, i do both so i will give a brief description of each subject to help you chose. Business studies, in year 12, focuses on the 4 key business function, being Operations, Finance, HR and Marketing. Essentially you are learning about how a business runs which might be helpful for you as you stated you want expand your business in the future. The finance aspect of business may also help with your accounting subject. Economics on the other hand basically looks at the running of an economy and focuses more on Australia in the global economy (topics such as trade) and Australia in the domestic realm (so what the government does- policies). I would agree with the person above to say that economics does challenge you a lot more and encourages you to think in quite a different way. Personally i prefer economics but i would say that business is also a very interesting subject.
I hope this is helpful and if you have any other questions relating to the two, feel free to leave them below