I'd take the antibiotics. Group B strep is very serious, can kill the baby, it seems worth some panic attacks (if they happen -- you'll be so busy during labor that you probably won't even notice the IV. They tape them down really firmly, they don't wiggle or anything). Ask the doctor about being able to perhaps take a Valium if you begin to have panic attacks.
How about doing the alternative options to get rid of it and have them retest you later. That's going to be what I do if I test positive.