Thanks all for your replies. I was under the impression that a postbac would be competitive to get in also. If I want to impress the med schools, then wouldn't be better to show them that I can handle some courses that med school students are taking as well?
Post-bacc programs (non-linkage) tend to be less competative than SMPs. Mainly because post-bacc programs usually have plenty of space since you take classes with other undergrads. SMPs on the other hand have less space, and they want someone who is more likely to survive the rigorous med school curriculum offered by the programs.
True, med school classes may imply that you will do well in med school. This is why SMPs are great. BUT, that may also depend on which school you are applying too. It is always up to the school to determine if "med school" classes are sufficiently rigorous enough. UCLA and University of Washington told me that they will look at med school courses on a case by case basis. These classes were taken at UC Davis School of Medicine, so not too shabby.
Ultimately, as Sundarban1 said, med school classes are still considered as GRADUATE level, therefore these DO NOT affect your undergraduate GPA. Therefore if that is what you are trying to do (e.g., improve your UG GPA), then I would suggest doing post-bacc. For me, i've taken numerous classes through our medical school due to my PhD requirements. But our director of admissions has always pushed me towards taking more upper division undergrad classes to boost my GPA. Therefore at least in terms of the University of California, undergrad GPA vs. grad GPA (regardless of med school classes or not) is more important.
Your original question suggests that you do have a <3.0 GPA, therefore it behooves you to know that some schools also screen GPA. The cut-off is around a 3.0-3.2, therefore you may not even get a secondary application if you are screened out.
Lastly, SMPs, post-baccs and whatever else tend to want students that have "high probability" of getting into med school. They do this to enhance their reputation. For example, a person with a 3.0 GPA who maintains a 4.0 GPA in the program will be in a better position than a person who starts out with a 2.8 GPA despite maintaining a 4.0 in the same program. Additionally all of these programs will have a variety of students that are applying.
