NASS v.s. CVW
I did not attend NASS, but I did attend SLS. I would imagine that they are extremely similar.
First of all CVW's and NASS are great opportunities for candidates to receive exposure to the academy. I believe all of those who receive an appointment should gain some short of exposure prior to I-day in order to relax the culture shock. If you are given the chance to participate in either program, take it!
That being said I enjoyed the CVW much better than my SLS experience for the fact that I feel I gained more from it. CVW is similar to job shadowing; funny cause USNA is training you for your future job as an officer right
Anyways, the CVW allows you to be "dragged" by a plebe for approximately 40 hours. during these forty hours you will wake up at the crack of dawn, attend actual academic classes of your plebe, watch the sport/intramural practice of your plebe, and most importantly
RECEIVE LOTS OF TIME TO ASK ABOUT LIFE AS A PLEBE. Nobody can tell you better what life is like at USNA than someone who lives at USNA. You get the opportunity to watch what your daily life as a plebe is like.
Now, SLS/NASS is much more like a summer camp. Its still great to go but it is certainly an entire different atmosphere from a CVW.
Personally, the CVW was a huge factor for me in deciding USNA was my first choice. When I went in October, I had an LOA in hand from USNA and USAFA. After spending those few days observing and interacting with the brigade, I knew I would be making the correct selection in USNA. I just felt the right vibe the entire time I was on the yard.