The short answer is both. Which is why many engineering degrees cover both topics to a greater or lesser degree.
Accountancy is very good for making you realise that there is more to the cost of a product than just the cost of raw materials. It also reinforces the need for good record keeping in a project (eg. keeping a record of how many hours have you spent working on a project).
Economics is good for looking out how the cost of an item is also influenced by market dynamics (eg. how many similar products are on the market at the same time and how much demand for that kind of product).
The long and short of it is that I would recommend, given the opportunity that you should try to do both. If I had had to choose between the two I would personally do Accountancy because it is something you can use sooner in your career as an engineer.
