The answer depends on you and what your weaknesses are. For instance Northwestern Prep's focus is on the SAT/ACT prep and the Candidate Fitness Assessment (CFA). The reason for this focus is because after high school, the biggest part of the whole candidate score that you can change is you standardized test scores and your CFA score. Now the downside is that none of their classes can transfer as college credit, but since it is only a semester long, you start college after winter break at another university. Now the military junior colleges, MMI, NMMI, and VFMA, have a slightly different approach. They do have SAT/ACT prep, but it probably is not as rigorous as the NWP model. They also have physical training programs to help with the CFA. What they do offer is college level courses that MAY transfer to another college. I say MAY because not all colleges accept transfer credits from all junior colleges, but these junior colleges will tell you which colleges will accept the credit and the required grade for transfer. Military junior colleges will be an introduction to the military lifestyle which could help indoctrinate you to what to expect at a SA. Now Marion Military sends about 30 candidates to West Point and a good number to Coast Guard since they use it for their scholars program, so if you want to be with a cohort of kids headed to a certain academy, that may influence your decision. NWP sends most of their students to USAFA. The military junior colleges also have ROTC which can open up an additional nomination slot if you join. Also, talk to your service academy admissions officer, they may recommend a certain course of action. Now if you feel you only need to improve you test scores and you are disciplined, you may just want to sign up for a Kaplan like test improvement course, sign up with a local trainer to improve your CFA, and go to a local university to take Calculus, Chemistry, College Level English, and College Level History. You have to be very disciplined and focused to do this route, because nobody is invested in you attaining an appointment. Remember all the preps make their living on getting candidates into an academy, so they have a vested interest in your success. Also, look at the letter of recommendation requirement. Getting letters from a university professor may be difficult as they will have no idea what you are doing.
I have some experience with this as I did not get into my academy after high school and went to a civilian college on an ROTC scholarship. The PMS I had at my college was an academy grad and really wanted me to go, so he would push me to get the requirements complete. If he was not like that, I am not sure I would have gotten an appointment as I was a typical 18-year old. I also wore my ROTC uniform to academy MOC interviews and received two MOC nominations (Senator and Representative).
Flash forward to having a son who just missed acceptance this year. I had to move after the end of his sophomore year in high school to another state. It was very difficult for him to get involved as a leader in any clubs or activities, since everybody had a two year head start in establishing relationships. We chose MMI, because we know he needed test prep and also strong letters of recommendations that he may not have been able to receive as the new kid in high school. He feels he is ready for college work, so wants to take college classes. He also wants the one full year bonding experience with other like minded individuals with common goals. We are in a area where there is not much military, so showing up to an MOC interview in a uniform will set him apart from other candidates. Finally, we asked the admission officer and that is the course they recommended.
So you need to do a self evaluation.