From personal experience, I found Biology harder simply due to the vast amount of material you have to memorise, particularly with AQA. If you want an A, you need to start revising immediately.
Chemistry is more conceptually challenging but there's far less material to learn. Just make sure you understand the concepts to the point where you can explain them to someone else, do lots of practice paper questions from the start and prepare well for the practicals. I personally enjoyed Chem far more than Biology.