Person who got A in both psychology and sociology at AS here. I recommend you do psychology.
Personally, I found both to be enjoyable. However, sociology is less 'clear-cut' than psychology; I was a bit confused about what to write in sociology essays, but in psychology, it was all nice, easy, and logical. I can tell you exactly how I got A in psychology, but I have no idea how I even passed sociology, and since you are a science person, I believe you may find psychology easier because all you need to pass it is logic and a good memory. (Or you can just make up your own studies and hope you get away with it, some people I know did that and they were fine.)
Do not worry about the 20-marker and approaches (or theories, if you want to call them that, though they are always referred to as approaches). The approaches are logical and/or easy to understand, so you will have no problem with them. The 20-marker is extremely easy: the introduction is just you making your point by using two studies which show two opposing sides of the argument and giving your judgement, the body contains 4 PEEL paragraphs out of which 1 should be an ethical/social/economic issue, and the conclusion is just you repeating how your examples support your opinion. At AS, psychology has less marks in analysis than sociology, meaning that instead of trying to think up an analysis for sociology, you can just memorise your examples and write an okay, not too detailed analysis for psychology (yes, that is enough for you to get full marks). You barely need to practise psychology essays, really, while in sociology, there may always be something you are forgetting, not to mention the essay structure for sociology is harder.
Psychology is overall easy as there is only a single 20-marker in the exams (this goes at least for WJEC Eduqas psychology). All the other questions only have up to... 10 or 12 marks, if I remember correctly. Most questions have under 10 marks. In the second paper (research methods), the question with the most marks is a 10-marker while everything else does not have an essay form and gives few marks (for example, you would be asked to define a term or write two advantages of a certain sampling method, which cannot be worth more than 4-6 marks at most). Everything is extremely simple.
On the other hand, (at least OCR) sociology has 20-markers in every paper. (Its 20-markers are longer, too; I was instructed to write 6 PEEL paragraphs in the first exam, to exemplify, and my second paper's 20-marker had 7 PEEL paragraphs. This was a bit of an overkill thanks to my teacher, but 5 PEEL paragraphs seems to be the standard, which is more than in psychology anyway.) While they are not astonishingly difficult, if 20-mark essays stress you out in exams, you will be more stressed if you take sociology.
Expanding on what someone else said, it is true that sociology has a lot more content. You will be busy with your hard science subjects, so it would be less of a strain to take psychology. I also found that in contrast to what another person said, I had more studies to memorise in sociology than in psychology. Psychology may seem like the one in which you have more studies to memorise since you are given 8 studies and 3 ethical/social/economic issue per debate in the book (again, at least for WJEC Eduqas it is like this). Even so, in the exam, you only need 5 studies and 1 ethical/social/economic issue per debate! This means you can just memorise 6 out of the 11 examples you are given in the book, which is everything you need to write a good essay. Needing just 6 out of the 11 examples represents about half of the examples the book gives you. Also, the ethical issues are no-brainers; once you learn the handful of major ethical issues, you can use them easily even if you do not know a study (you can even make up a study and get away with it, in fact). This is far from the case in sociology. Basically, psychology looks like it has a lot of studies for you to memorise on the surface, but you only need about half of the studies written in the book, so there is no need for you to bother with the rest, thus resulting in sociology actually being the one with more studies to remember.
In addition, Russell Group universities prefer psychology over sociology. I know at least at Bristol, I once saw some kind of statistics which showed how for politics and international relations, people who studied government and politics, English literature, and psychology were more likely to get an offer than those who had the same subjects, but with sociology instead of psychology.
As for which one is better suited for law, the answer is most likely neither. However, psychology is much more popular choice for law than sociology (I looked it up and in 2012, 28.6% of law applicants did psychology while 18.7% did sociology).
Overall, everyone I know who did psychology and sociology loved psychology waaay more than sociology.
OP, I also did not swap sociology for history because of friends. I now wish I had taken history instead. Do not let your friends be a factor in what A-level choices you make, for it can turn out to be a huge mistake.
Good luck!