Would a knowledge of accounting or economics be more useful to help running a business/ business management as a career in the future?
I would say it really depends on the type/scale of the business, but I would opt for accounting, simply because if you have the ability to interpret financial data it would make it much easier to understand the inner-workings of the business.
BUT in saying that it could be argued that economics may give you an advantage in gauging the current and future economic climates and therefore being able to prepare the business better for what lies ahead, taking advantage of any opportunities and protecting against possible losses.