Hi, I am in Y12 and I am currently taking A-levels in maths, French, computer science and chemistry (one will become an AS), so I can definitely help you out with your decision. I have decided to drop chemistry to AS because I have found chemistry to be quite different to GCSE (A-levels quite a bit different to GCSEs in my experience). Chemistry is definitely a lot harder than computer science so far, but it really depends which you enjoy more, and if your department for those subjects are good at your school. Chemistry has a lot of content compared to computer science and you have to get your head around that content because it can be quite confusing at time, on top of this, you need to be able to apply and learn all of this to exam questions and experiments. I would say computer science is a better option, but it is also depends how good your teachers are, even though A-levels are more independent than at GCSE because you need good teachers to explain things well at A-level chemistry otherwise you can easily become left behind and you have to keep on top of it (unfortunately, my teachers aren't the best for chemistry).