Sociology
Advantages: it's not terribly difficult, the paper is shorter than most (50 minutes for Unit 1), freedom of argument.
Disadvantages: a lot of names to remember along with their concepts, a lot to remember, and you need to evaluate a lot.
Psychology
Advantages: the course is more straightforward, with more concise knowledge needed, as well as some room for debate (ethics).
Disadvantages: mathematical/statistical elements might deter you, again remembering names/concepts, and a pretty long paper.
At A2
Sociology expands on the concepts and theorists you learn at AS and makes you apply it in different ways with new perspectives (e.g. at AS you might know Functionalism, Marxism, Feminism etc but at A2 you have to know Weberianism etc.) and linking it to new circumstances and issues - e.g. doing education at AS could be linked to crime in A2 if you talk about anti-school subcultures being more likely to break the law.
Psychology takes what you know already from AS and expands upon it with new topics that might seemingly contradict old ones, hence bringing new amounts of debate into the mix. For example, one study on depression at AS might be counteracted with a conflicting account at A2 - as well as research methods being an integral part of the course both at AS and A2.
At university and as a career
Sociology is arguably the weaker one, with very few sectors specifically related to it - which is true of all Arts degrees. You would be looking at roughly £21,000 starting wage, and would learn skills of communication, reasoning and debating, as well as an understanding of people and current issues - politics is a common field graduates go on to.
Psychology however does require more work - unless you're aiming to teach, you will generally need to go on and get an MSc in Psychology, or even a PhD if you want to work in psychological research. Psychology graduates have a larger field of careers to choose from, including law, politics and government, clinical psychiatry, neuroscience research* and teaching. However, if you choose this path you would be expected to be doing a science as well, usually biology or chemistry. You'll be using maths as well.
* neuroscience research at university and beyond often requires a science A Level, usually in chemistry.
Which should you pick?
It will depend on the person and their preferences - if you feel you are more scientifically minded, go for psychology. If you prefer debate and no set answer, or have an interest in politics and current affairs, go for sociology. And if you can't choose, take both! I'm currently doing A2 in both and I love them equally. For a career in either though, psychology is the safer bet, although it takes more work than sociology. If you asked me to pick though, I would say Psychology simply because of the nature of it being more fun.