Classes are exactly the same for pre-reqs. Some schools who have medical and dental schools have both students in many of the same classes their first two years of medical or dental school....
Dentists have to like teeth... a lot...
I can only give you pros and cons of medicine, I don't know that much about dentistry other than that you likely will be able to keep bankers hours, make decent money (but less earning potential than physicians), and still have decent job security/placement
as for going into medicine, pros: more specialties, higher earning potential, more schools, cons: higher risk (malpractice), longer training, more BS with insurance/et al...
these aren't inclusive lists, and you should shadow both people in order to get a feel for both. That way you will have more information to make the best decision for you.