There are two reasons to do a postbac:
1. To complete medical school prereqs after finishing an undergrad degree
2. To improve undergrad GPAs with further undergrad study
(these reasons may overlap)
There is one reason to do an SMP:
1. Your undergrad GPAs are too low, or your application is too weak, to get you into medical school.
If you haven't completed the prereqs and taken the MCAT yet, then you can't (or at least shouldn't) do an SMP. You can always do a postbac.
If your GPAs are low and you can improve them with additional undergrad study, then do more undergrad. It's cheaper and less risky than an SMP. After about 6 years, GPAs don't move anymore.
If your GPA is over 3.0 but under 3.4, and your MCAT is 30+, then do an SMP.
If your GPA is under 3.0, and you can't mathematically improve it (too much undergrad), then get your MCAT over 35 and do an SMP.
If your MCAT is low but your GPA is fine, retake the MCAT.
Any good SMP is going to be at least 2/3 as hard as medical school.
No postbac is going to be more than 1/3 as hard as medical school.
The first two years of medical school are roughly equivalent to a 25-hour all-science courseload, which roughly translates to a 75-hour study week. This has been described as drinking from a firehose.
There's a summary of SMP linkage in the 2009 SMP list at the top of this forum. Some structured postbacs (for those who still need to complete prereqs) have linkages; unstructured postbacs do not.
Based on your questions, I recommend that you spend a bunch of time reading the FAQs and other threads at the top of this forum.
Best of luck to you.