As mentioned, there aren’t huge differences between the exam boards. They all follow the same criteria set by Ofqual and Department for Education - however, AQA has some extra content here and there, but they are not difficult to learn.
The one huge difference between AQA and OCR is that in AQA Paper 3, there is a 25 mark essay you will need to write. AQA is the only exam board that does this. They give you 2 questions, and you can pick one of them to answer, and you’re required to write about at least 4 topics across the AQA spec.
Grade boundaries are roughly similar for both exam boards.
Biology is a hard A Level regardless. I do AQA, and whilst I love learning the content, the exam questions can be a pain, especially as mark schemes are super specific. I’m not sure how OCR is.
I would just like to add: I think for Biology, the resources are a bit limited compared to AQA Chemistry. If you do end up taking AQA Biology, I would advise asking your sixth form/college to buy the Biology with Olivia Notes - they are around £50 for the whole AQA course I think. Also, Mrs Estruch on YouTube is a great teacher - I don’t watch her often but my classmates do and they say she’s very helpful! For Chemistry, I would recommend notes from the chemrevise website, chemguide and the YouTube channels Allery Chemistry and Elliot Rintoul.