Which school is better for your A-Level options? - you probably know the teachers for your school for your chosen subject (and therefore know if they're good or bad), whereas the other school is more of a gamble. Do both schools offer your chosen subjects?
Benefits of staying: - the school knows you, and therefore the teachers will be able to write your personal statement for university more easily - you're familiar with it - Year 11 to 12 is already a big change - you have friends there
Benefits of moving: - change is often good - you'll have practice at making new friends / getting used to a new environment (important for the future) - the school may be better at your subjects
Ultimately you're the only person who can make this decision Good luck