As someone headed first for the PhD ...
A medical degree does not prepare you for basic sciences research. Many MD's who do research actually do clinical/clinically-oriented research, for which you would still need a residency. Quite frankly, as someone coming out of medical school you would probably not be a strong candidate for good post-docs. You definitely won't be publishing anything while you're in medical school, your lab technique will be rusty and/or outdated, and a lot of the effort you put into medical school would be a complete waste (like clerkships, maybe gross anatomy, all of the patient-interaction stuff, the list goes on ...). Even if you get a decent post-doc, you'll have a steep learning curve for both scientific knowledge and laboratory techniques.
You would also be at a huge disadvantage financially. Any decent PhD program includes funding for full tuition, health insurance, and living stipend; our debt is solely from undergrad. As a non-clinical researcher, you would face a lower earning potential and have a higher debt than your peers.
If you're not sure about research vs. clinical practice, why don't you try a terminal masters? 2 years, funded, and you'll finish with publications and experience. You'll improve your strength as either an MD or PhD applicant. You'll also get a much more realistic idea of research than what you could have gotten as an undergrad.
