If you have the opportunity, see if you can go work in a dentists office, or even shadow for a while. I worked in Oral surgery for a few years and was bored to tears, so obviously it's not for me. But, your best chance is to see what the daily routine is for a dentist, and even check out some of the dental specialties to see if that is something that you can do for the rest of your life.
The one thing that I did notice is that it was alot of the same thing day in and day out, and I'm not saying that a MD/DO dosn't have a monotonous routine at times also, but like Pete said above, medicine will give you a broader scope of practice overall, while dentistry will pretty much be working in peoples mouths every day.
Also, just wanted to add that if you are interested in both and are not in a big hurry, you could do both! My ENT is a MD/DDS and has a particular interest in maxillofacial surgery and facial skeletal deformities. Very smart guy!