I have heard so many different things regarding this subject. As far as which is more "beneficial" to attend depends on you as a candidate. Weigh the pro's and con's... Do you have a first choice? If it is USMA, then great, go to SLS and Boys State.
The reason I'm saying this is because there are other ways to visit Annapolis first hand that are, in my opinion, a much much better way to see the the Academy. I believe West Point only offeres visit weekends to candidates with appointments; However, I went to a USNA Candidate Visit Weekend which meant I got to live in Bancroft hall with a plebe (w/2 roommates) for 3 days. (go to all of their classes, get amazing behind the scenes tours, eat with them, get "chewed out" with them, I even got to drive a Yard Patrol boat.. It was incredible, and more beneficial to me than the NASS was) I was a Junior and I hadn't even fully completed my application. If I were you I would contact my regional director to see if I qualified to go to one of these, that way you can go to boys state instead of NASS.
When I was a Junior, I e-mailed my regional director for USMA to kinda "get on his radar" and ask him how I could improve. His only reply was to enhance my application by going to Girls State. I was thinking "what!? that's it!?" but sterlingmom is right- they actually do regard it as a very impressive aspect of a candidates application.
Best of luck! I hope this helps!