Put the job description for your current role, or the one you are chasing, on the desk in front of you and read only the verbs. If they are mostly implement, administer, support and process, the PHR is the credential that matches your work. If they are mostly design, develop, evaluate and lead, it is the SPHR. HRCI built its two flagship credentials on exactly that split, then stacked different experience thresholds, content outlines and renewal rules on top of it.

That is the short answer to the PHR vs SPHR question, and it holds for most readers. The rest of this article shows the evidence behind it and the cases where it needs a second look. It covers how HRCI's own outlines use those verbs, how the two exams compare on paper, the eligibility thresholds that keep them apart, where the content outlines overlap and where they part company, who should skip the PHR altogether, and the signs that it is time to book.
One thing this article does not try to do is sell you the bigger credential. The SPHR is not a better PHR. It is a different measurement, aimed at a different job, and a candidate who picks it for the letters rather than the work usually finds that out in the exam room.