The sole remaining "A" lens, as far as I know, is the tack-sharp 50mm f/2.0
normal lens (probably made in China, but has great build quality).  There
may be a couple of cheap zoom lenses, but for this discussion they don't
really matter.

These are dark days for classic lenses.  Everything seems to be
plastic-barreled zoom lenses that are made by robots and weigh less than a
half-pound.  The good stuff is available only used.  The FA-Limited lenses
are autofocus designs, and they don't have that classic feel when manually
focused.  Plus they cost an arm and a leg, relative to the classic manual
focus designs.

There is little left except the Leica R lenses, still made in the classic
way--with metal barrels and lots of handwork--but they are priced out of
reach, and there is speculation that Leica may withdraw the R system due to
poor sales, focusing instead on the M and the digital camera lines.  Who
pays thousands for a lens if there is no presumption that service is
available for a long time?

So, if we agree that the future for classic lenses is in the used market,
not newly-manufactured ones, then the question is which of the two
manufacturers has a broader supply on the used market.  If I were going to
select one brand over the other, forgetting that I am already heavily
invested in Pentax M42, I would go with Nikon.

Of course, if the OP is looking for only one or two lenses to complete his
kit, and is not concerned over future used availability, I'd say he could go
either way.  Although an FM-2, metal, would be more to my taste than a ZX-M,
plastic.  But my personal biases in favor of metal cameras may not reflect
what the OP wants.