Full mouth restoration using All-on-4 technique. I am wondering if Crystal Ultra or Zirconia is the answer?

There are several choices for hybrids when considering a full arch dental implant restoration. These include porcelain to metal, acrylic on titanium frame, Trilor with Crystal Ultra, PMMA with frame and Zirconia just to name a few. The strongest material is monolithic zirconia as long as there is enough thickness in the material. However, some say it is too opaque compared to some of the other materials or the sound when the teeth come together sounds like glass over glass. The best is for your dental provider identify what meets your needs and what has the best long term outcome in your particular case.

The PMMA prosthesis (teeth) is designed as a temporary solution, and should never be used for than short term as it is prone to breaking.
Zirconia is considered an ideal material but there is some concern that it may be too strong of a material (counterintuitive concept) and as such can be prone to breaking (especially with clenchers/grinders). It ultimately depends on how the teeth are made by the dental lab.....an older concept is a set of teeth made out of one piece of zirconia milled by CAD/CAM. While this produces a very esthetic result, if the zirconia breaks (I’ve seen it happen), the entire set of teeth needs to be remade.
A better choice is a titanium substructure, with zirconia layered onto the titanium and individual porcelain crowns (Emax).... it is an incredibly esthetic result, it is no more expensive than a single piece zirconia prosthesis (teeth), and if a tooth ever breaks, it is much cheaper to replace a single tooth.
Also, you need to insist on a nightguard being made along with your final set of teeth. Without one, you risk fracture of your final set of teeth.  

If your concerned about durability then Zirconia is the choice. It is much stronger than what I have seen about Crystal Ultra. Zirconia monolithic is the strongest versus layered Zirconia. The  Crystal Ultra appears to be more cosmetic but it is not nearly as durable as Zirconia. Just make sure you get the cosmetics you want.