I do both so I'll let you know what I think
Psychology:
Pros - Very varied topics (you do 11 with AQA, including research methods, so if you have 1 topic you don't like it isn't a major part of the course (research methods not counting, but we'll come back to that).
The longest essay at AS is 12 marks and at A2 it's 16 marks, which isn't that long so if you aren't stellar at essay's it isn't a problem.
Has a scientific basis (if you like that of course, this may be a con if it isn't your thing but the science isn't too hard).
The class book we use (
Cons - A large focus on Research Methods (It's minimum 25% across all three papers, and paper 2 has a section solely on Research Methods which is worth 48 marks, double the length of any other section). The stuff covered is also much harder then Sociology, with things like statistic testing, Sociology research methods is much easier.
Many people think it will be all things like 'mind reading' or mental health discussions, it isn't. As a result many people in my school were disappointed with it and dropped it.
Sociology:
Pros - Essay's can be worth up to 30 marks, the paper is out of 80 marks, so if essay's are your thing you'll be good.
Research methods is much easier then in Psychology, you won't need to revise research methods in Sociology for very long.
Less scientific, so if science isn't your thing you'll be happy here.
Cons - There are 6 main sections, including research methods and a section called 'theories and methods, so if you find a section you don't like you'll be with it for a long time.
Paper 2 is the 'optional topics' paper, so you have 8 options to choose from and your school will pick 2 of those 8, if you get an unpopular topic there might not be as many resources available. Anecdotal evidence I know, but one of my Sociology teachers told us how one year they did the topic 'The Media' for A2 and there were no extra resources to help them, so they switched to Beliefs in Society the next year.
If you use the Ken Brown book as your class book it is very wordy and awful to revise from, so buy the CGP book, it's a life saver.