Im not sure which way will guarantee acceptance to medical school. The advantage that post-bac programs have is that there are may set up for Pre-meds to finish their requirements. They have advisors and programs to help you with the admissions process. There might not be the same kind of support in a Grad program. At the same time a Masters degree does look impressive. You also have to keep in mind that med school may not accept you until you have finished your graduate degree which takes about 2 years or more.
I took the option of taking post-bac biology classes to boost my GPA. Im not in a post-bac program so Im my own advisor. I can pick any classes I want, but sometimes wish I had the support of other classmates in the similar situation as there would be in a defined post-bac program.