I've just finished year 13 and did psychology and sociology. Although both essay-based subjects, essay writing skills are more important for sociology as psychology essays are more structured so if you want a subject that shows you can do essay writing too then sociology may be best. However, doing biology will probably mean you will particularly enjoy the biopsychology (AQA) chapter in psychology and you will probably find the most difficult chapter (in my opinion) more easy. Although both have research methods, this is based more around essays for sociology (20 mark questions for AQA) whereas psychology involves more maths, including percentages, averages, ratios and stats tests, which you may prefer if you prefer science/maths based subjects. Both are quite content-heavy but as long as you look over the content regularly then both are manageable. I found both really interesting due to their relevance though I personally found that sociology was more interesting overall while psychology content became more interesting in year 2
Hope this helps and I'm happy to answer any specific questions you have about either subject