I've never used them, but the nt1 is obviously the old design and is noisier,
the nt1a is made with surface mount devices (SMD) components, which has lowered the self noise. However, for resons that I admittedly have a very sketchy knowledge of, you won't find SMD components in real high quality gear. Why, when it could lower the noise? Quality control is one thing, also beware of some companies that sell stuff quoting low noise floors and low THD figures.
Anyway, after hearing what I've heard, I won't get one.