Biology is a lot better than Chemistry at IB. The course involves a lot more practicals, and I personally found the Biology course to be extremely interesting. I'd still choose it over Chemistry in a flash. My best mate, who did chemistry, absolutely detested it. At standard level in particular, biology is significantly easier than chemistry.
As for Geography Vs. Economics . . . either one is pretty broad and useful. You do have a larger coursework component in Geography, if that helps you to make up your mind. My personal choice is [naturally] Geography, because it's considerably broader than economics and is often offered as Geography And Economics anyway.