As mentioned before, it's all about whether you're interested in the course. Try and have a look around bos and see if there are any notes on prelim first topic Economics. If there are, have a look along with any other topics you can get your hands on. If they interest you, go for it.
Most the kids at my school, as well as my Maths teacher, don't like Biology because they think it's boring. Economics in year 11 is actually pretty damn good. You'll also more than likely look at the GFC and what's caused it, etc. So just choose it if you're interested in it.
Preliminary is easy catch-up. Don't stress over it.
EDIT: If you want an ATAR and don't really care how you get it consider if your bio class is smart and will maintain high marks. At least compared to your current Economics class. If you just want to do something enjoyable, go with Economics.