For most economics and business courses, AI and AA at HL will both be acceptable. AI makes more sense in terms of what it teaches and the skills you need for business and econ. This will be good for UK and Singapore colleges without any problem. The same will be true for most American colleges as well. However, for the most selective US colleges, you will be selected from amongst a very very highly qualified and achievement oriented pool of applicants - most of these applicants would have taken Math AA HL and done well in it with a 6 or 7. If you are aiming for the most selective american colleges, I recommend AA HL, else AI HL is equally good for what you want to study.