I currently take both biology and psychology. To take computer science at uni id say both are as good as each other as choices but if i had to go with one id say biology. I find biology taking up more of my revision time than psychology. Both have a lot of content to remember but, I find psychology to have more content that is basically common sense and that majority of the content is not hard to grasp/comprehend. Biology has a lot more content that may take a while to wrap your head around especially the processes that you will need to know inside and out and in detail. Exam wise, they both have three, two hour exams at the end and biology has practical assessments as an element as well. Psychology exams are a lot more lenient in the mark schemes and forgiving if you describe the meaning of a term you cant remember whereas, biology mark schemes are very specific in key terms and phrases you have to include so it can be frustrating. Biology paper 3 includes a 25 mark essay and the rest of questions will mainly be between 1 and maybe 7 or 8 marks, Psychology paper include mainly questions of 1,2,3,4,6,8 marks maybe a 10 or 12 mark question for 'designing a study' and essay questions are 16 marks which are generally 1-4 a paper. Id look at a few requirements for uni courses, if they require you to study another science alongside your physics, psychology is not always included as a science. I find psychology to be a much more enjoyable subject overall which can motivate me to revise and I find biology to have certain aspects that are a real bore, for me i like genetics but cant stand plants. Id definitely look at the specification for each subject on which ever exam boards website your course may be.