I doubt your undergrad biology program involved medical school related classes. I know most biology degrees involve ecology, and zoology classes in addition to biomedical classes. But most biology majors don't involve histology, pathophysiology, human anatomy with cadavers, embryology, pharmacology and other classes that are offered in medical school itself.
For the purpose of getting into medical school, it doesn't matter whether you get a Masters or a Post-bacc. What's more important is taking the proper type of classes. You need to take classes that are also offerred in medical school. Take classes like histology, pharmacology, immunology, pathophysiology, biochemistry and other classes that are offerred in medical school. Whether you find these classes in a Masters or a Post-bacc doesn't matter. Just pick a program you like that offers these clases and sign up.
If you should do a Masters program, make sure it is non-thesis oriented. Thesis centered Masters programs involve a lot of research classes. Reseach classes are easy A's and admission committees know this. They won't take your grad school GPA seriously if 1/2 of your curriculum is involved with research classes. Non-thesis related Masters programs involve regular classes that are based on exams. This is one reason why most people on this forum advise against picking a Masters over a post-bacc. Post-bacc doesn't waste time with research classes while most masters programs have some research aspect to them. There is nothing wrong with research. But if you are trying to raise your GPA, you need to spend most of your time with real classes as opposed to research classes where you are given instant A's.
My post- bacc is better than most Masters/graduate programs because of the type of classes that are offered. They only emphasize biomedical classes so our biology doesn't waste time with anything outside the human body. And all of my grades are legit. I don't have any fru fru research classes that I got an A in just for showing up.
I wouldn't waste time retaking what you have already taken. So check out the post-bacc and Masters programs of many schools. Use your common sense. If you have taken 80% of what is offered at a post-bacc at one school,, then don't bother applying to that one.. Every post-bacc is different so do your homework. They don't all offer the same classes.