In a nutshell, the SCJP (OCJP) 6 allows you to get certified by taking one exam instead of two but uses an older version of Java. If your goal is to get certified as quickly/for the least amount of money, the OCJP 6 is your best bet. If your goal is to learn Java/get a job, the OCAJP and OCPJP 7 exams will be more beneficial.
If you intend to use OCPJP as a prerequisite certification to other certification path (ex. OCMJD), then I'll settle to taking OCPJP6, saves me a lot of money. And also the consideration of preparation materials available for OCPJP6 compared to 7.