There’s a good question. They are both doctorates, they are both advanced degrees but there are many more differences. There are a number of SLPD programs across the U.S. These are relatively new compared to the Ph.D. and they meet a specific demand.
The American Speech-Language-Hearing Association (Asha) has information comparing the two degrees. The SLPD is a clinical doctorate. It is designed for SLPs who hold the MA/MS and have their CCCs. Most of the programs I’ve looked at offer it as a two-year degree. Some are in person, some on-line, some are hybrid. Ph.D. programs are generally not on-line programs.
The goal of the SLPD is to help prepare SLPs for more advanced roles in their current positions. It is for SLPs who want more training in the profession, EBP, understanding research, supervision and leadership. SLPDs may become clinic directors, special education directors, supervisors and so on. I think this is the right degree if you are interested in digging deeper into the practices of the profession. A 2 year MA degree covers the scope of practice but it doesn’t give you deep knowledge in any one area because there is so much to know! So, an SLPD would be a way to develop some more advanced knowledge and to stretch in terms of your role.
Some people have asked me if an SLPD will be sufficient for a tenure track job. Unfortunately, the answer is almost always no. It doesn’t mean that it can’t or hasn’t happened, but that the SLPD is not the training needed for a tenure track job. For that you need a Ph.D.
The Ph.D. (in CSD or a related field) is a research degree (more on this here and here). The training focuses on doing research and teaching. This is one reason the Ph.D. takes longer (4 or 5 years full time). The Ph.D. is typically fully funded. In the Ph.D. you are expected to learn to do research and to start to develop your own line of research. Ph.D. programs are generally face-to-face rather than on-line because it’s about learning to be an academic. I usually think of this as learning a language and a culture. You are expected to publish, present at conferences, and to defend your ideas and work. For example, my goal for my students is to have about 2-3 (more is better) publications in peer-reviewed journals by the time they’re on the market. This helps them to position themselves in terms of an area of expertise, and shows that they are “tenurable.” You also work on development of teaching competencies. In many programs you will take a course on adult pedagogy and TA or teach a class (or both).
All in all the two degrees are pretty different and lead to different roles. I think there is room for both types of degrees. If you want a doctorate you have many good options depending on what you want to do with the degree.