If you ask someone in the psychology world how people with their PhDs differ from those with MDs, you may get an answer like "MDs can prescribe medication, whereas PhDs cannot." That is true. Another difference in general is that MDs make more money in the US.
MDs are consider by many to be the "real doctors" because they can help with real medical problems. That too is true. I certainly don't refer to myself as "Dr. Kelly" in any context other than academic ones because people might get the wrong idea that I could jump in and help in the event of a broken foot or migraine headache.
All that sounds pretty bad for the PhD. But here's the most essential difference between the two degrees: PhDs advance knowledge, whereas MDs merely apply existing knowledge. Unlike the MD who does not need to produce any original research, the person earning a PhD must produce original research and write it up in a thesis or dissertation. Then a committee of experts must deem that thesis as offering an acceptable advancement of knowledge before the PhD is conferred. It typically takes a couple of years longer to earn the PhD. Part of the reason it takes so long is that the person earning the PhD is being trained on how to think critically about existing knowledge, and it can take a while to find one's niche and fill a gap in the knowledge base.
This means that if you yourself want to make important scientific discoveries and then tell the world about them, you will be much better prepared by getting a PhD than an MD. You also will be much better prepared to criticize studies you read about in virtually any field because you will be trained in critical thinking and writing. MDs typically aren't as qualified as PhDs to evaluate research studies and their significance.
If you are deciding which degree is right for you, ask yourself if you will be content with applying the knowledge you learn (MD) from other people, or if you would like to get in on the action of making the discoveries yourself (PhD). For instance, would you like to be one of the scientists who are figuring out how to reverse the aging process (PhD)? Would you like to see if giving aging mice a particular the enzyme (one that you discover) makes their hair shiny again and restores their fertility (PhD)? Or would you be content giving your future medical patients the proper dose of the medications that arise from this research and then seeing the signs of youth return in your patients (MD)? These are the kinds of questions that college students everywhere should be asking themselves, and yet I have never seen them do so.
This difference in training also means that if you want to know what the cutting-edge knowledge is in a given field, you have to ask a PhD in that field, not an MD. So for instance, let's say you or your mate is having trouble getting pregnant. If you just ask your local obstetrician or gynecologist what the cutting edge discoveries are regarding fertility, that MD is not likely to know. That MD can give you fertility treatments that he or she has learned about and tried with other patients.
The upshot of my message is this: We need both kinds of people, those who apply existing knowledge (such as the MD does in the medical field) and those who advance it (PhDs). But if you think a PhD is less qualified than an MD when it comes to having cutting-edge knowledge, you have that backwards.