Here's a few things to look at for these SMCs:
1. Visit each of them (preferably an overnight). You will find that one feels like "home".
2. Outside of the military, the alumni networks for the SMCs are more regional (e.g., Norwich is mainly England, while VMI is mid-Atlantic, and The Citadel is in the southeast). Of course, there are grads and clubs from each school dispersed everywhere, but statistically, grads tend to end up in the school where the SMC resides. So, if you plan to have a post-military career in South Carolina, statistically, you would have a better alumni network if you attended The Citadel than Norwich (converse is true if Boston-area was your home).
3. Weather is a factor. If you're a snowbird and love to ski, Norwich is your best bet. If you love hot weather, The Citadel might be best.
4. Civilians on campus. Norwich has them (although bulk is Corps of Cadets), while others do not. Not sure if this is important to you.
5. Look at funding sources and cost of attendance. This can be important down the road.
6. For military career, there is no advantage from either SMC. The one thing I can say, though, is that if you are in the National Guard in one of the states like VA, VT, or SC, it's likely better to come from the state where the local SMC is located. Active duty military is different.