All the specifications are quite similar. For example in the sciences/math they normally all contain the same topics, and the content is probably 90% the same. The exams generally take similar format too, although there are some differences. It depends on the subjects really, but the grade boundaries are always scaled so that it is always just as 'hard' or 'easy' to get each grade.
Sometimes the quality of their textbooks varies, so if your exam board has a bad textbook I would recommend having a look on amazon for another book of the same subject/exam board.