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The answer to your question depends on what you really look for with a certification. If what you need is a programmer certification, go for OCPJP 6. If you need some up-to-date certification and you have the time it takes, go for the way to OCPJP 7 which requires OCA as a prerequisite.
Sorry that I maybe misinterpreting what you were trying to say. When you say 'exercise' do you mean mock exams or assignments? Answering the question about how the real exam differs from ExamLab, the most important thing to mention is the real exam is not that tough as ExamLab. ExamLab questions are made in a way that 'practices' you to figure out the tricky parts, so you can easily spot them in any significance of the real exam. We have an article on ExamLab that describes why ExamLab questions are made in that way and the best way to achieve the optimal advantage of practicing with ExamLab questions.