the best thing to do when it comes to choosing a levels is really to pick based on what you personally enjoy and would have enough motivation to revise - i enjoy my subjects but can hardly bring myself to revise them so I can't imagine what I'd be like if I didn't care about them! sure chemistry is notoriously a 'hard a level', but equally biology is content heavy and you need to be very specific about your answers, and physics is equally as difficult. personally I only take biology and I find it so fascinating, but ultimately it's your choice