Of course I could be completly mistaken…..but it seems that is you want to boost your GPA then go for the post-bac…..but if your GPA is already great and you just want to look "different" then maybe a graduate program could work
Nope, that's definitely true....a postbacc (doing undergrad classes after receiving your BA) is for those who were not premed in college and still need to take the prereqs, and for those who need to improve their undergrad gpa. By its nature, a true postbacc cannot be a masters degree because you're taking undergrad, not grad classes. At least at some schools you can get a "certificate" for your postbacc...which I think is just some way to saying you completed the program, but otherwise has no real value (as far as I know). I did a postbacc and it took 2 years, but I was doing the prereqs for the first time. For most of the programs I don't think it's that hard to get in, but a lot of people drop out along the way (decide medicine isn't for them, or that they despise science, etc. etc.).
There are special masters programs that also have the aim of improving your application to med school, and as far as I know you take grad/med school classes, and is I think often seen as a way to prove that you can handle harder/upper level coursework....this seems more valuable for those who might have had a rocky start in the sciences, but did well enough that they can't take things over or already took upper level undergrad sciences...but that's just my impression.
Both are solid choices, it just depends on what you need to improve on.
There's an entire forum devoted to postbaccs/master programs....check it out.