I just sat for and passed the PHR exam in December. You have to have at least 2 years of exempt level HR experience to take the PHR. They recommend at least 2-4 years experience for the PHR and at least 4-6 years experience for the SPHR. The PHR is more focused on the daily operations of HR and the SPHR is very heavily focused on the strategic business partner role. Recertification requirements have also played a role in a lot of people's decisions as to which exam to take. Both certifications require 60 credits in each 3 year period. The SPHR requires a specific number of recertification credits in strategic management wheras the PHR has no requirement as to which specific areas your credits are in. I think it really depends on your role in your organization and you goals in the near future. Many people will take the PHR and then take the SPHR several years later when their career has progressed to the executive/director level. We did have some people in our study group who just took the SPHR and had never done the PHR. Depends entirely on you.
I participated in my local SHRM chapter's certification study group, which I can't recommend enough. You can purchase the SHRM Learning System at a discounted rate. They have HR professionals who are already certified come to lead modules in which they have particular expertise. Even if you have a lot of experience, I think everyone can benefit from the study groups. If nothing else, the study group keeps you motivated and on track.
I used the SHRM Learning System as my primary study tool. All of the content was great and reflected the content of the exam, but the test questions were a very poor representation of the actual exam. I also used Anne Bogardus's SPHR/PHR Test Prep book (on Amazon.com for about $35) and found the practice test questions to be much closer to the actual exam. I would also recommend taking HRCI's Assessment exams (2 exams for $70) which you could use as a pre and post test to your studying.
Good Luck!!!