If possible, do the ATPL. The depth of knowledge you can achieve is much more than the CPL. Can/should you give all of this knowledge to a PPL'er? No, but it definitely helps your confidence that you have the background. Why restrict yourself? Maybe, for what ever reason, you can do something else that needs APTL. It is a grind and the value of learning something via QBs is questionable, however, do the theory once and nobody can take it from you.