Zirconia Crowns Vs Emax Crowns?

Hands down, Emax will look better than Zirconia.  Emax is a porcelain, Zirconia is actually a white metal covered over with porcelain.  Because the white metal on Zirconia crowns does not allow light to pass through it as naturally as the all ceramic Emax, Zirconia crowns often look flat and unnatural.  Technically, you should not get any staining on the surface of either provided the zirconia is covered with porcelain.  However, if the zirconia does not have a porcelain outer shell, you can expect it to stain somewhat depending on the kind of "bad things" you do (i.e. smoking, tea or coffee, cola, cigar, etc.).

In my professional opinion, a cosmetic dentist with 35 years experience, Emax would be the best restoration for most front crowns or porcelain veneers, it has the benefits of great fit, strength, longevity and appearance over Zirconia.

Sorry you are not happy with the recent P2Z crowns you had done.  The changes you mentioned are unusual as porcelain restorations such as porcelain to zirconium or emax do not change in appearance, especially in a few months.  The most esthetic material is Empress in my experience.  Its limitation is it has a greater potential to chip or break compared to P2Z or Emax.  Emax has improved greatly in the last couple of years in how excellent labs are able to make it look and it is certainly more durable.