I do all three sciences with OCR. I think you will find biology easier than chemistry, but they are both great subjects to take. With biology, as long as you understand the topics and learn the contents, you should be able to get a good grade. With chemistry, once you understand it, the main thing will be to practice the topics by doing past papers. I think the biggest difference between GCSE and A-level exams, is that for A-levels the mark schemes are so much more specific.