SMP = many cases gives you a Master degree. You get a new clean nice graduate GPA column (if you get good GPA in the program). It is considered a graduate level course, so it is a better way to prove that one can handle the rigor of dental school ( if one has an irreparable undergraduate GPA). Some programs have formal or informal linkages to dental school. Cons: usually more expensive than postbac programs.
Postbac = usually when people say postbac, they refer to additional undergrad courses after obtaining your bachelors. This option is good, if one's GPA is around 3.1~3.3 and want additional courses to improve their chances. This option also has formal or informal linkage (for example: SFSU - dental postbac has linkage with UoP or UCSF). Also, usually cheaper than SMPs.
I'm currently making a list of master programs (SMPs), so if you want it, PM me.