I would highly reccommend the Alumni Association. $540 over four years isn't that much. You don't see much of the Alumni Association till you're a firstie (I'm talking straight from experience right now).
Over this past year, they have supported our class at all our major milestones and events (service selections, orders night, etc.) They have held many functions and socials this past semester to support us in graduation. This week, we received out life member packets, filled with goodies and contacts lists. The Alumni Association is your link to the "extended Brigade." They publish the free monthly "Shipmate" magazine to members which gives graduates news about the Academy and also ways to keep in touch with friends. The very few people who didn't sign up for the membership are now regretting it. They can still sign up, but it costs more money.
They way I see it is that I'm going to be an alumnus of a school that is very selective and prestigous so I will join its membership.
As for USNI, don't know much about it, but they offered the graduating class free three year memberships this year so everyone took advantage of it. USNI is an organization dedicated to the puclication of naval related matter. They also pusblish the "Proceedings" magazine which is an open forum on national defense. People write their own opinions and articles for broad publishment. It's a pretty good read.