I've just finished year 13 and did both psychology and sociology. Athough both content heavy and essay based, psychology essays are more structured whereas sociology essays are more flowy. Psychology is more scientific (particularly biopsychology) and has a bit of maths. I personally found sociology more interesting whereas for psychology the content is more interesting in year two than year one, though the relevance of the content makes both interesting
Hope this helps and I'm happy to answer any specific questions you have