A Level or GCSE?
GCSE, I would say cgp. Simplifies, learn from the book, everythign you need to know
A Level, I would say the exam board textbook. Has all of the content and with examples that are often used in exams. The questions are specific and there lots of stretch and challenge work and it gives reference to wider reading (which you should absolutely do, as i comes up in exams). CGP is a bit basic for A Level, as the exams arent about learning and repeating, its about a deeper understanding and being able to apply your knowledge.