Just to clarify, it is NESA that sets the minimum unit number requirement (in this case, 12 units in year 11). School simply adhere to NESA rules.
To answer your question, you may wish to consider a number of factors in making an informed decision. First, while Economics does indeed scale higher than Business Studies, scaling alone should not be the main reason for choosing a subject. Favourable results and ATARs can be achieved with both subjects (and any subject combination for that matter), as long as you regularly study and prepare for your assessment tasks/exams.
Second, you should take the subject that you believe you would be more interested in. From your post, it seems that, based on your experience in Stage 5 Commerce, you would be more interested in Business Studies.
Third, you should also consider your performance, that is, the subject that you believe you would be able to achieve better results in. The fact that your school generally performs well in both subjects sounds promising.
Alternatively, you could take both subjects, leading to a total of 14 units, assuming your school does not place restrictions on the maximum number of units. This would allow you to experience both subjects at no cost throughout year 11 (because none of your year 11 results will count towards your HSC results or ATAR) and subsequently determine the best subject combination to have as you approach year 12.
Bonne chance ! I hope this helps!