The NS drives have a five year warranty, vs. a three year warranty for the AS drives.
That corresponds roughly to the comparative MTBF: 1,200K hrs. for the NS vs. 750K hrs. for the AS.
I couldn't find the warranty for the AS drives at seagate.com, but Amazon has it here:http://www.amazon.com/Seagate-Barracuda-7200RPM-Internal-ST31000528AS-Bare/dp/B00272NHOK
The NS drives are much more expensive than the corresponding AS drives, from what I have seen.
A 1TB NS drive weighs 672g vs. 622g for a 1TB AS drive.
The load rating for the NS is 3w vs. 2w for the AS.
The drives are not the same. I wouldn't try to use the PCB's interchangeably between the NS and the AS. But you might, and you might even get away with it.