OP -- Does Son have an Offer of Appointment to USMMA, or an LOA ? If just an LOA, what is the contingency ? Until he has an Offer of Appointment in hand, neither is a sure thing, and this is largely a theoretical discussion.
If DS has an offer of Appointment to USMMA -- I am the biggest supporter of NAPS, and love Navy, but I would recommend taking the bird in hand and go to USMMA. I like the USMMA curriculum, and wish I knew more about it when I was applying.. (One of my HS classmates went to USMMA, and until he go in, I had never heard of it !). As you aware -- he can commission into any Branch of the Service, and if he wants to go SWO, I would even suggest that the sea time is actually an advantage over a USNA grad.