Here's what you do, Cms204.
First, don't spend even a microsecond worrying about which school is better at this point. Apply to USNA like it is your NUMBER ONE school. Apply to USMMA like it is your NUMBER ONE school. Apply to Grove City College like it is your NUMBER ONE school. Apply to all of the other colleges like they are your NUMBER ONE school. At this point, you have no idea who is interested in you and who isn't. Until you do know who wants you, there is absolutely nothing to talk about and it is a COMPLETE waste of time to try to do so.
Second, once you RECEIVE offers of admission in March/April, that is when you have something to think about. DO NOT GIVE IN TO TEMPTATION AND JUMP AT THE FIRST OPPORTUNITY THAT COMES ALONG. You may receive an offer from USNA, but "do not fire until you see the whites of their eyes" no matter how tempting it is. If you want to make a good leader someday, anywhere in life, you must learn to evaluate all possible courses of action before committing to either one. This is so even if you know in your heart of hearts that you want to proceed along a certain path. This Forum is filled, regrettably, with students who receive an appointment from a SA and jump on that offer like a rabid animal, even though other opportunities have not been presented yet. That is NOT the kind of decisionmaking that you will learn as a military officer. Rather, you should stay calm, calculated, and then make your decision after all facts have been properly evaluated. Sure, in the military, there are plenty of occasions where you have to simply react quickly based on imperfect facts (like if you find yourself in the middle of the kill-zone in an ambush), but you are NOT in that situation -- our college admissions system is basically set up so that students have the entire month of April to decide what they want to do.
If you receive NO offers, then your options are clear. You have none. However, if you receive multiple offers, only then can you weigh the pros and cons of each. Included in your offer from Grove City will be a financial aid package -- weigh that against the others, which are free! Also, then -- and only then -- do you consider aglages' point about civilian versus military. For now, though, continue your workout routine like you are going to USNA/USMMA, get 3Q'd at each, and get all of your info together. If you receive the BFE, you should spend an overnight at each SA to really get a feel for the place. Don't base your decision on the fact that one SA has a better website than the other! And spend an overnight at Grove City if offered admission as well. Again, once you have all your facts together and have researched this at length, then you make a decision and you NEVER look back. You should have the entire month of April to make your decision. Students are not required to make decisions until May 1 -- for ALL colleges, including the SAs.
I would NOT share anything with Grove City at this point about USNA or USMMA, especially when you haven't been appointed to either one yet. Only if they ask, which they won't, then you should disclose it. There is no benefit to you from the perspective of Grove City admissions -- they are NOT going to say "Gee, if this kid is good enough for USNA, he's good enough for us!" The only possible benefit would be for the financial aid decision. But you should wait on that until you've hooked them on the line. Once they have extended an offer to you, THEN you can say "Gee, have we explored everything here in terms of grants/scholarships? I'm currently deciding between Grove City and the FREE Service Academy." Just know, though, that there are about 1000 kids on the waiting list wanting to take your spot at Grove City, and Grove City knows this.
Finally, I do want to address the point made by blackhawkmom, because her advice is very good. As you will learn in the military, NEVER leave your flanks exposed. If you do get accepted to Grove City and to USNA, pay the deposit at Grove City and, on the morning of I-Day, call Grove City (or have your parents do this) and say, "Sorry, but I've decided to go to USNA." This may be dirty pool, but it is prudent. blackhawkmom's point that you could get injured between the date you accepted the appointment and I-Day is sound. Other problems could arise as well that would have the same effect of letting an appointment slip through your fingers.
Good luck. Grove City is a very good school, by the way. Inexpensive, too!!