Edexcel is not harder or easier than AQA or indeed any other exam board. Each exam board creates their exams based on the national curriculum and can have different styles of questions. This means that they can award marks in slightly different ways and have different accompanying revision material. It is worth exploring multiple exam boards in order to decide the best exam board for your students. There's lots of guidance and advice for teachers and SLT on the Third Space Learning blog including an article on the differences between the three main exam boards.