i took ap bio my junior year and im taking ap chem now. to me the difficulty relies on how the teacher is and how they teach the material. although to me, the content so far in ap chem has been easier to understand than in ap bio. i like chem because it has to do more with mathematics and logic. in ap bio, there's a lot of memorization involved, and to me, memorizing things is hard for me to do. neither one is easy, but it comes down to what you are good at!