I wouldn't state it quite that way. For those who are unsure whether USNA is right for them, attending NASS can help with that decision. While it is not nearly as rigorous a program as Plebe Summer, it does include elements of what life will be like as a mid. For quite a few people, NASS either cements their desire to attend or makes them sure they don't want to apply.
CVW provides a different type of snapshot in that the entire brigade is there, you attend real classes, get to see all four classes of mids, etc. That said, for the most part, you're observing, not doing and sometimes that can skew your views/decisions.
Finally, if you attend NASS and do well there, it can slightly help your chances for admission. However, the "bump" you get is probably less than what you'd get from Boy's State. Not attending NASS does not hurt you.
The above said, if you cannot make another NASS session, I would probably opt for Boy's State and ask for a CVW unless you are very undecided about whether USNA is right for you. In that case, I would probably do NASS.