There is aboout a 40% difference in content and questions, and the CPL is of a lower academic standard, with 13 exams instead of 14. And because there are less questions your chances of not passing are increased, as each question counts for a higher percentage.
If you know you won't need the ATP in the future, then don't bother with it. if you are simply using the FI as a means of building hours, then consider it, but you will need to get the IR within 36 months. Also, if you get the CPL and the IR, then want to upgrade to the ATP, you will have to do the majority of the exams again because of the mismatch between syllabuses. In which case you may as well do the ATP in the beginning.