Let me summarize the CGAS Prep schools (I'm an instructor at MMI)
First, all students take courses prescribed by the academy. Each takes Math (often through Calculus), Composition, Chemistry, and Physics. CGAS can take validation exams to transfer credit into the academy.
MMI: Two-year military junior college, credits transfer. All students members of a Corps of Cadets. Varsity sports of Track, Cross-Country, Tennis, Men's Basketball, Baseball, Softball. Tuition paid by the academy with MMI financial aid. CG pay and allowances go to room, board, and uniforms, but often with cash left over.
GMC (Georgia Military College): Two year military junior college with a high school/prep component. College credit does transfer. CGAS are members of the corps of cadets, but many students are not members (Think Texas A&M). Varsity sports available, including Baseball, Softball, Cross-country, and Football (See GMC website for full list). Tuition paid by the academy with GMC financial aid. Like MMI, CG pay and allowances go to room, board, and uniforms, with cash left over.
NAPS (Naval Academy Prep School): US Navy school to provide a prep year, primarily for prior enlisted Sailors and Marines to get up to speed for university level courses. All students are active duty and subject to US Navy regulations. Courses are not accredited and do not transfer college credit. CGAS receive base pay only as meals and housing are provided by the US Government. Varsity sports available in all DI sports that USNA participates in.
UMass-Boston: Accredited University. CGAS are paid full benefits but do not wear uniforms. Specifically for students majoring in engineering. College credit does transfer.
UConn-Avery Pt: Accredited University. CGAS paid, but housing is provided at CGA. CGAS wear uniforms on campus (both at UConn and CGA) but are NOT members of the Corps of Cadets at CGA. Primarily for students in management majors.
If you're selected for CGAS, you can put in for the school you desire, but will be assigned by the academy. I do not know what criteria are used for the selection. As any veteran will tell you, your desires are listened to but the needs of the service take priority.
If you go as a self prep, you cannot apply to NAPS. NAPS is for sponsored students only. GMC and MMI both have interaction with CGA admissions and have programs to help candidates become more competitive.
UMass and UConn are relatively new to CGAS, so I'm not as familiar with their programs as with the others.