I did both Psych and Bio for A-level, and psych was new to me too. As someone else commented, the curriculum is designed to be accessible to students who've yet to study the subject so there's nothing to worry about in that regard. Biology does however assume prior knowledge (science GCSE is a core so students would've studied it), The level of detail and knowledge for bio A level vs GCSE is quite a step up (I remember spending my first lesson not knowing what on earth she was talking about!) so it will feel like a new subject, similar to psychology. I that respect I think they are evenly matched. Either way, after a few months, you'll settle into your subjects and they won't feel so new to you.
Personally, I preferred Bio over Psych (its not Darren Brown type stuff) but I still refer to knowledge I gained from both in my life now. As for you, look into the course content and absolutely go with whatever subject you think you'd enjoy more and be more motivated to work for.