Mountain Mom:
Congrats to your son. As noted the admissions process to all four Service Academies that require Congressional Nominations are very similar. The differences btween the process are relatively small and for USAFA, USMA, and USNA are mainly driven but what the admissions ranking process weights and considers more and less. Each of those SAF have lots of active folks who can and likely have provided you feedback, etc. on similar questions and I wouldn't begin to add to that as they are not "in my wheelhouse."
Those of us who know and love USMMA would of course agree with your naiscent conclusion - USMMA is absolutely a gem. The differences between USMMA and the other three academies that require nominations with regard to admissions are rooted in the legal statuates that govern the operation of USMMA and to some degree, I believe certain key aspects of that CFR is driven by the relatevely small entering Class Size. Rather than ~1,200 entering Freshmen ("Plebe Candidates") - USMMA generally has an entering class of 275 - 310. So instead of each Memeber of Congress (MOC) having a "prinicple nominee" and how that all works, each member of Congress (and ONLY Congressmen and Senators) can nominate up to 10 candidates - from anywhere in their State - that meet the other criteria for an appointment to the USMMA. Those nominees then compete first for the legally allocated number of spots for their State in each entering class and after that they then are placed into a national pool to compete for any and all unused spots on that level. I confess that at this point I don't understand the actual mechanics as to how this works with the implementation of early and rolling admissions, but it does and the implementation of those processes has been done in a manner that meets the requirements of the applicable CFR that you can find specifically to the Nomination process here (46 CFR 310.53):
http://www.law.cornell.edu/cfr/text/46/310.53
and to the overall top level requirements "46 CFR 310, Subpart C - Admission and Training of Midshipmen at the United States Merchant Marine Academy" here:
http://www.law.cornell.edu/cfr/text/46/310/subpart-C.
Life at and after USMMA is as noted different than life at each of the other four Academies (I'll include USCGA here as well) and/or ROTC programs etc. To me one of the biggest difference between USMMA (as well as USCGA) and either USAFA, USMA, and USNA is really rooted in the difference between a truly small college and a midsized/larger college. In general USMMA and USCGA graduates know almost 100% of their classmates when they graduate as well as many of their companymates on either side of their graduation year by a year or two. This is not generally true of USNA. USAFA, or USMA graduates and classmates. To be sure at all the Academies the esprit d' corps is there and the bond between classmates is strong but it may or may not b rooted in actual direct knowledge and shared expereinces that you had while literally looking at each other while you attended the school. Further athletics - USMMA fields 19+ Division III sports teams and has an overall student body of ~950 (of which only ~700 are on campus at any time). So while the athletics programs at all the Academies always amazes me - how does Navy legitimately compete with Notre Dame every year on a football field, etc. - the extreme liklihood is while you are at USMMA if you want it you will have the chance to play DIII sports and you will certainly know some folks who do. To some degree everyone at an Academy is a bit of a "jock" and to the opposite side of things - no one is - awfully hard to "be a jock" and graduate a four year course of study with only three years on campus, etc.
As noted and debated elsewhere here - USMMA is a Service vice Military Academy and it's principle focus is the training of young men and woman to be leaders in the Maritime Industry. I'm not trying to start a big debate on that but I trust when you look at all the information, along with the cited CFRs you'll see what I'm referring to. Many, many folks come to USMMA from landlocked places and become successful Mariniers who have a love for the sea that exceeds those who've grown up next to the ocean, etc. That said, it's much easier to understand why you want to get through any of the rigorous courses of study that getting through any of the Academies requires if you have a desire to be what they are primarily training you to be. I'm sure that will weigh into your son's decision.
They are all great options and they all lead to great opportunities. For myself and my own DS who recently graduated USMMA was for sure the way to go - we'd both tell your son that. For various friends and colleagues of ours, the other options your son may also have were the way they would tell him was for them and they wouldn't change a thing. I'm not trying to advocate one way or the other just trying to highlight a couple of differences in both the admissions process and what I think is a truly substanative difference between USMMA and the other three Academies that require Congressional Nominations.
Good luck to you and your son in navigating through the part of the journey.