預科生物/...econ??

Biology is hard to you if you don't get good memory on those specific terms while Economics is easier as it requires more understanding than simply memorising. Go to the one you are more interested in, and you will do better. Nothing is easy if you don't put effort therein. If you can't do one subject, you have no choice not to do the other, and likewise, if you can't do both, choose the one which is easier for you. It seems that Economics is your better choice.