This reply is a little late, but I just noticed this thread and would love to contribute. I have just recently finished my first semester at NWP and I am now attending a local community college.
I do not know much about Greystone, but if I am correct I believe Greystone was founded by a former Prepster (that is what they call students who attend NWP). But I could be wrong.
tug_boat did a great job of explaining NWP. It is a stressful academic and physical environment. The school is at 5,000ft, but that does not stop them from giving you hard workouts everyday. I was in good physical shape going into NWP but I have improved greatly since I first started there.
The academics are tough as well. You have four classes (on top of your physical training class) which include: English, Physics, Chemistry, and Math. You only have around 3-4 hours each day to study for those classes, not to mention the 20 word vocabulary tests every day. So they are big on time management.
This school prepares you for success, not just in the academies, but anywhere you decide to go. It is a small school where you will get to know everyone very closely (this includes the Durbecks, who have good connections).