A PhD means you really like research and you want to pursue it further and it would qualify you to teach a basic science course in dental school (besides those at the collegiate level). Most people that have a PhD have a research component to their career (e.g. a periodontist here at Penn regularly sees patients, is an active professor in the microbiology department, and has a lab). An MD means that you are nuts about school and went on to medical school after dental school (that's right, another 3-4 years plus how ever many it takes to complete whatever residency). Just because you have an MD doesn't mean you are qualified to perform oral surgery or practice oral medicine (and conversely not every OMFS program/residency, and certainly not every Oral Medicine residency, hands out an MD in addition to the certificate of completion for that residency, so you don't necessarily have to have the initials MD behind your name to practice either of them) as you have to successfully complete residencies for both of them in order to practice.