3rd year the same as GCSE?
I'm doing it as A-Level, and Economics is very different from Business Studies (which I did at GCSE). It's much more about the economy as a whole rather than focusing on one business, I thought they would be pretty similar but I was wrong. If you're genuinely interested in what's happening in the economy at the moment, like unemployment, social welfare and inflation, then Economics would be a good choice. If you're more interested in advertising, types of businesses (franchises, sole traders etc.) and how they are ran by management, go for business studies.
Although, there is the stigma that's apparently associated with subjects ending with 'studies', although obviously do the one your more interested in and think you would do better in then what would look good on a university application.