It's way too early to start compromising on your dream career. Do your due diligence on both professions, then forge ahead towards that profession. A less scrupulous option would be to take the dental position, but keep your options open for med. If later you want to do med, then just drop the dental program and pursue medical school.
Dentistry and medicine are two very different beasts even though they share many similarities. Shadow, inquire about, and research your options.
Best of luck!