The three porcelain systems you mentioned differ in chemical composition. Procera, the oldest is, is aluminum oxide with conventional feldspathic porcelain fired over the alu-oxide core. It is indicated in single tooth applications only and has largely been replaced by Emax which is lithium disilicate. This material has been out for a few years and is rapidly becoming the industry standard. It is very esthetic and has excellent strength. The hardest porcelain for dental applications is Zirconium oxide. You may have heard of BruxZir crowns. I saw a promotional video where one of these crowns was driven into a 2x4 block of wood with a hammer and didn't fracture. It is indicated in bridgework and people who severely grind their teeth. Although the new translucent zirconium is more esthetic Emax is probably the best blend of strength and beauty!