I took both Sociology and Psychology at AS, and honestly I preferred Psychology. It didn't help that my Sociology teacher taught the course in the dullest and most uninspiring way ever, but I just didn't really get it really.
Psychology is great if you can memorize solid facts and studies, but there's not much room for just making things up on the spot - you have to be totally accurate. There's lot to learn, so you need to dedicate quite a bit of time to revise the content, though I found the course rather interesting. The essays are a lot more straightforward to write, too.
On the other hand, you can be quite explorative with Sociology. If you can hold an intelligent and well balanced argument backed up with studies, sociologists and key terms, I think you can afford to improvise a bit with the essays. They're certainly less solid than Psychology, and kind of subjective in terms of how to write the essays.
To be honest, I think either are fine on their own, but I would advise against taking both (which you're not doing anyway, which is good haha) as they both require lots of time and effort. Though, I totally BS'd the Sociology exams and still came out with a B, but worked incredibly hard to revise for the Psychology exams and came out with an A, so interpret that as you wish, haha.