I had the same problem too, ended up picking Psychology because I figured there would be elements of Biology within Psychology (i.e. neuroscience, psychological drugs, etc) but I ended up switching into Biology after a year.
My experience of Psychology was it just wasn't interesting to me. Yeah, documentaries about criminal psychology are interesting, but they're a very small component of your degree. The British Psychological Society (BPS) requires Psych degrees to have breadth as well as depth. So you'll be covering a lot of different things, from how to learn, how children develop, sport psychology, memory processes, etc (have a look at uni's lists of modules on offer). Additionally, the studies, to me, are quite basic e.g. a study showed children would copy an adult displaying aggression. It's kind of obvious through just making observations as you go about life and I don't think it warrants a proper, clinical study to prove something so simplistic.
Neuroscience was my favourite part, but I noticed all my Neuroscince lecturers weren't even from a Psychology-background, and instead had gone Biology/Biomed type degrees. So I decided to switch into Biology.
My experience with Biology has been positive. Mainly due to my uni giving a really large selection of modules so you can really tailor your degree (I keep away from all the plant biology modules, for example). Again, check the lists of modules on the uni's websites. Biology has a lot more contact hours (20-25hrs per week for me at the moment) compared to Psychology (around 7hrs per week). Again, this will vary uni-to-uni so contact head of admissions if you want more details about workloads and timetables. Biology has a heavier workload, too. Biology is roughly 70% exam and 30% coursework (essays, lab reports, online tests) whilst Psychology was 100% exams in first year. Biology is a lot more difficult and requires a lot more effort, but you get more contact hours in lectures and labs which aid understanding. You also have practicals every week which can be 2-4 hours long but are really interesting, you're using proper equipment and doing things like PCR that you will have only read about so far.
So to summarise, Biology is a lot more difficult but, in my opinion, a lot more rewarding and interesting to study. If you like getting your teeth into complex pathways then Biology is for you. Psychology is easy to understand but requires a lot of rote memorisation of studies and findings.
Quote me if you have any specific questions.