Well, the KSC is what's going on today, it's a rocket display, it's the other tourist traps in the neighborhood, it's a Imax theatre (ours is going to close in june:-( ). But it's also not obvious to see everything you could have seen, before, for security reasons.
The NASM is much more accessible, concentrated, inside, (also with an Imax theatre)the history of flight and spaceflight. I know it's possible to touch a moonrock. It's perhaps also the case at the KSC but I'm not sure.
In terms of documentation and books, I found the NASM more interesting than KSC which is more into toys.

Raoul