Modern porcelains look very lifelike now. I would always try to put all ceramic ( no metal ) crowns on front teeth. Zirconium is a lovely material but sometimes looks too white or opaque. I would recommend crowns such as Empress as they look more natural. However, all ceramic crowns require more space so more of your tooth has to be removed, we do still use metal based porcelain crowns in some cases as our dental technicians are very skilled at making natural looking teeth.
I hope this helps