Hello! So I am currently doing A2 Chemistry as well as Biology
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So biology hurm... This takes up a lot of time! No joke. The amount of content that you have to learn for biology is a lot. I achieved an A grade for biology at AS, which of course i am thankful of, and i was among 2 people of a class of 25 people to achieve an A-Grade. To be fairly honest i done about 16 hours of revision for biology a week, 2 hours on weekdays and a lot more on weekends, this included making notes, attempting past papers and of course the lovely old memorizing business! That's just an overview of how biology is, an oh before i forget, you have heard wrong. The step between GCSE biology and AS-Biology is far more greater than from GCSE Chemistry to AS-Chemistry, that is my opinion tho.
Chemistry - Well chemistry in my opinion is a lot easier than biology. I done about 8 hours revision a week for chemistry, which is half the amount i spent on biology. I achieved a B-Grade for chemistry this year, 3 marks of an A, which is not to bad since i done better in the harder unit and can just retake the easier unit, if i did spend more time on chemistry then i would have got an A. Chemistry involves a lot of maths. If you like maths this is great fun, if not then not very pleasing. It also involves a lot of practical work, WAYYY more than biology, which is pretty cool and fun. Notes on chemistry are easier to make on biology and the notes are not very content heavy.
As to your question about whether to carry on with bio or exchange for chemistry, i personally in your position would exchange biology for chemistry reasons for this are:
1) Universities value Chemistry more than biology.
2) Chemistry does not involve a lot of stress like biology.
3) Chemistry is more fun to learn.
4) You can pick from a wider range of course with chemistry.
However you have an opposite argument, if you stick with biology then:
1) You are more likely to get into a course which involves more human anatomy. But generally speaking you need A-level chemistry along with A-level biology to get into these type of courses. This is a very important point which you should consider.
Anyway, Those are just brief outlines. However the decision ultimately lies with you.