Black nitride color wears off the rails fairly quick but the treatment is there forever. Make sure gun is fitted and functioning perfect before getting it done cause it hardens the hell out of the surface.
PVD is supposed to be harder than chrome but it's applied so thin that it's probably a toss up with hard chrome on longevity. Which is a long time.
Some gun part manufacturers frown on nitride(high application temps.) and hard chrome (chemical reaction)
I nitride or pvd my builds unless customer requests hard chrome.
That's my 2 cents worth. Greg