Consider NWP a finishing school that has a highly focused semester that is geared towards maximizing improvement/showing the academies what they want on an application. They train specifically for the CFA, have a standardized course load that preps the candidate for the classes and shows the appropriate plebe-like courses for admissions to consider, and they specifically train and retest on the standardized tests. The staff there is focused on the academies and admission, not producing 2LTs for the reserve/national guard like some other prep schools. For those benefits, there are drawbacks - it is only one semester so you have to enroll somewhere for the spring and if you are not successful, you likely can't take those credits from the first semester with you.
MMI is a more traditional military college that is accredited and will give you an associates degree and a commission after two years. There is no real advantage to self-prepping here versus going to a normal college and participating in ROTC other than being immersed in the cadet lifestyle (which can be argued that there are other options for that as well at 4 year schools). Most of the academy offers that MMI and others attribute to their program to show success/draw in self-Prepsters who pay a lot of money comes from the AOG/Civil Prep/sponsored candidates. Merely going to one of these schools (even excelling) won't necessarily earn you a ticket to the dance.