psychology and sociology are seen as similar in levels of difficulty. i did a level psychology and didn't struggle to understand the content, it's the amount of content you have to remember which is probably the most difficult part of the subject. both are essay based/have extended writing questions and it'll depend on the exam board the amount you have to write though i believe sociology has longer mark questions. both would also probably demand regular revision throughout the year to ensure content remains fresh in your mind. pick the subject which interests you the most.