As someone who attended both NASS and this past fall CVW I will share a little bit about my experiences for the CO 28ers.
NASS is, as others said, a recruiting program designed to focus on representation from all over the States. There were a lot of Puerto Ricans, Hawaiians, etc. Just because they want these really distant people to get to experience the academy. The NASS curriculum and modules seems like it is really geared towards resembling some parts of Plebe summer. You are lead by rising 3/C cadets in a squad similar to Plebe Summer. You get some experiences from them that resemble Plebe Summer (without disclosing too much as to ruin some of the novelty of being along for the ride and not knowing whats next, also like PS). There were people I know who after NASS were no longer interested in USNA. Obviously, it is better they found out because if they couldn't do 5 days they were probably not a good match. But NASS has that energy. It's a good taste of the exhaustion, and occasionally, harassment that PS can offer. Just on a pint sized scale, all while giving the rising 3/C an opportunity to practice leadership. While there is an admissions process to get in, and I would say it is a good first step to be accepted, admissions consistently stressed that NASS was not a representation of future admissions for said class.
A CVW, on the contrary, is said reliably by admissions to be a good indicator of a promising file. Heck, Adm. Buck will tell you himself at a CVW that you are there for a reason because you are promising. A CVW is a very cut and dry exposure to the Ac year, as opposed to NASS might be to PS. You get a realistic experience of going to real classes with real MIDS and getting some exposure to yard activities on the daily basis. I was able to attend a sports practice during my CVW. This is a real view of USNA, not sugar coated, complete with early mornings with tired plebes. If you have drunk the cool-aid as I have, you will love it.
As others said, NASS is perhaps more a time for you to consider USNA and for USNA to evaluate you (in part by seeing if you continue to be interested), where as the CVW will likely play a large part in YOUR decision-making if you are appointed. I think mine will be instrumental in me giving Army WP a big GO NAVY!