Economics is a lot more relevant to the world I find, and no matter what, it'll always be useful to you. I personally haven't taken business studies - from what I've heard as a GCSE it was alright, but for A-level I'd avoid it and take economics over it, because it's got less of a reputation.
If you want to know what the subjects are about - economics is basically studying about the economy, how consumers and governments handle scarce resources, you'd do a bit on businesses and business behaviour, but it's much wider than that and looks at it on a much broader scale. Business studies is quite self explanatory really, not that I've taken it, but it sounds like an in-depth studying of businesses.
In short.... take economics.