Always a hard question to answer. If you think there are "high chances" of denial at USNA...you may not be competitive at USMMA. Most USMMA appointees are competitive at the other Service Academies also. Not all get appointments, but they are competitive. A large number have 4 year ROTC scholarships or other "full ride" type scholarships. A few have multiple SA appointments.
At the campus visit for those with appointments on 4/21, Admiral Helis made the comment to the 100+ appointees in attendance that he knew most of them have multiple offers they were considering...or they would not have appointments. This was the day to convince them that USMMA was the place to go. I met one young lady who had just visited West Point and she was trying to decide which appointment to take. I only spoke to about 10 appointees directly that day, but all were very impressive young men and women. I think 3 had already accepted their appointments...the remaining had some tough decisions to make in the coming week.
So if you are thinking of USMMA, then you likely need to change things so that you are competitive at USNA and competitive for a 4 year ROTC scholarship. Then you will also be competitive for USMMA.