I took both for AS and plan on dropping Sociology for A level.
Psychology has more scientific content (naturally) and as someone also planning to take a science to uni, I find it is more helpful both in applying and in the content of the course.
Sociology I find a very relaxing lesson to be honest. It's, in my opinion, easier but there is a lot of content and not much of each unit actually comes up in the exam (AQA) so you learn all of it when some bits may not be asked about.
As you can probably guess I prefer Psychology, but opinions aside it is likely to be looked onto better than Sociology by unis and has more useful skills (stats and reasearch methods) than Sociology and both have essays.
Either would be a good mix with the sciences though, so take which one you think you'll enjoy more.