Zirconia Crowns Vs Emax Crowns?

Hi:Emax is a good option.  Zirconium is more durable for people that are clencher or grinders.  If you do not have this habit, you can have layered Emax, whereby the lab layers porcelain on the Emax for better aesthetic.Best,Dr. Maddahi

Thank you for your question. In general both are strong, where Zirconia is stronger. Emax usually looks better.It is not the material as much though that determines the look. It is a mastership and experience of a doctor and his technician, which makes you say "Wow!" when you smile and look in the mirrow.
Both materials can be used in a way that looks very natural. Think of it as a piece of jewerly. Would you want to buy a chip ring for your loved one or a custom made piece made by a master jeweler?

Cosmetically, for me in 2017, eMax is the winner and is stronger than other ceramics other than Zirconia. 
Zirconia is not a great cosmetic option unless strength is more important than the cosmetic outcome as it is a dull flat tooth shade that has to be stained or overlayed with eMax or porcelain to be acceptable.  
Most importantly, It will come down to who actually makes your crown that will give it the most cosmetic result so it is really up to the dentist and what they prefer and works best in their hands and/or with their lab technician and their abilities.

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.