I am an RN so I have had the greatest exposure to medical. Also I do enjoy the diversity of the human body. I honestly found the mouth and teeth boring, but I've never really done much with mouths, except very basic oral care. I wonder if I'm missing something.
I've always loved helping others and human physiology, so medical seemed like my first choice. However, a lot of small things pushed me towards dental.
For me, I am a very family oriented person. I want to get married and have a family before before im ~33, as well as have a little fun before that. This is pretty tough as a MD student, because of having to do 4 years of med school plus 3+ years of residency. I also love the consistent schedule of dentistry, simple hours, no working on weekends, whereas for (generally), for most of your life as a doctor you will be on call. i've had friends who complained that their dads had to leave in the middle of dinner or at 4am to go to the ER or something. Having a simple 9-5, 4 days/week schedule as a dentist appeals to me greatly. Money is also a huge factor I wont lie, but both medical and dental doctors make a generous amount. Both have business aspects to them, but I'm sure opening up an MD office is MUCH more difficult than a dental office.
The job is also a lot chiller as a dentist form what I've seen. You don't have to stress constantly that someone's life is in your hands like MD's do. One of the biggest turn offs from MD for me was the thought that a patient might die in my car, and I don't know how I'd handle that and it scares me a lot. On the flip side, MD's save lives, which dentists generally don't do.
So if you're in it for helping people, you can't go wrong with either. Obviously as an MD you're making more significant changes to someone's life than a dentist will.
I haven't gone to dental school yet, but from what I hear, the new DMD programs share similar courses to MD programs, so you learn similar to MD style courses.
All in all, they are completely different lifestyles. if you enjoy the stressfulness of being an MD, the payoff is great since you can be saving lives. If want to relax and enjoy life with a family etc..., it'll be much easier as a dentist. Don't get me wrong, MD's can do it just as much as a dentist can, but as a dentist it is much easier imo