I did the CPL exams on the grounds that I was going to be an FI and had no desire to get an IR. At the time (2 years ago) there was very little decent CPL distance learning material avaialble, so I (and most of my course) ended up using ATPL references in any case, particularly the Oxford notes which are very good. Unfortunately sorting the 'not required for CPL' info from the ATPL texts can be very tough, and you end up studying into the ATPL syllabus almost as a matter of course.
If I was starting over I would definitely study for the ATPL. It is slightly more work, but IMHO maybe only 30% more. I do understand that the Cranfield CATS guys now have decent CPL course notes, as well as some excellent instructors (Stuart Smith in particular) who really know their stuff.