You're in Yr 11 right?
Well it depends on the person, which they'd do better at etc. Things to consider:
BIOLOGY
Pro is that you do the two other sciences, so pracs should be cakewalks for you
I assume you like science to do three
Con is that 3 sciences might be an overkill unless you really love science
And if you'd perform similarly in the two, it scales not as well as eco
ECONOMICS
Pro is that it'd break up your maths/sciences
You are doing ee1, so I'd assume you'd tackle the subject well
Neutral point: I'd say it's more content heavy than bio, a good or bad thing depending on your memory
Negatives you may be more a math/science person
Eco is content and content-can become dry as it's not broken up with pracs etc. like other subjects
They are both good subjects, and even though I dropped eco, I didn't have anything against the subject. It's a good subject but I like bio more. But I was trying to be a little impartial for you, and I trust you'd do well with either set.