Hi Ben, I am Marvel
Firstly, congratulation on your AS results, many people will kill for the result you have achieved.
Personally, I don't really think there will be much of an difference between A2 Biology and Chemistry, as they are both highly-respected subject, certainly in the eyes of RG university. However, please check the specific subject requirements of your desired university, just to be sure
In terms of difficulties, I have done both A2 Biology and Chemistry and I will be honest with you.... In regards to the step-up from AS to A2, the gap in Biology is relatively minor compared with Chemistry, because the level of difficulties are pretty similar, there is just higher volume to learn and also the answering style is also pretty similar. (With less recall and more application ofc)
In chemistry, the step-up from AS to A2 is pretty major, both in terms of difficulties and volume of knowledge. And in chemistry there is more emphasis on understanding than chemistry, as A2 chemistry involve a lot of chemical calculations/theories, which you need to understand in order to apply it in the exam. In Biology, there is more element of pure recalls in Biology than chemistry. (It go without saying that you need to understand biological concepts as well, in order to apply it in exams, but I am sure you already know that) (Be aware that there is an reduction in pure recall in A2 Biology in comparison with AS Biology)
But if you willing to work hard, I am sure you will be successful in whichever subject/subject combo you choice to do. --Marvel