My dentist says a zirconium crown is as good as a gold crown. What is the difference?

Gold lasts longer. Gold requires less tooth removal, is soft and forgiving on the opposing teeth, and the margins are less likely to decay. I regularly see nice gold crowns that are older than I am; a zirconia crown will simply not last that long. Zirconia is a fantastic super hard material, but requires a lot of tooth removal. It also is very abrasive once the glaze rubs off; it will wear down your opposing teeth.
Priorities? A zirconium dioxide or zirconia crown is an extremely hard tooth colored ceramic material. Gold crowns are indestructible but not esthetic. Zirconia crowns approach gold crowns in strength but can fracture. They are an excellent compromise between strength and esthetics. Nowadays most people do not want to have a gold crown in an area that shows when smiling. Others do not care.
Actually, similar. Zirconium is extremely hard. Recent research shows that it actually wears like the natural tooth. There is no reason why more tooth needs to be removed that the same amount that is done for a gold crown. It is hard to discredit gold in any way. When well done, it lasts a long time. The two advantages of zirconium are: it can be less sensitive and it has more of a white/tooth colored look.
Cost. Each crown has its benefit. A gold crown is still the best dental material. With that said ziconia crowns are very strong and do look like teeth. A disadvantage to gold is the color and the cost. An advantage to gold is less tooth structure needs to be reduced for the most part. Hope this helps, .
Color and strength. Zirconium is a white metal, and is very strong. It is also a better match to tooth shades. However, zirconia requires more tooth reduction than gold does for material strength. In my practice, we rarely use gold because there are other strong and very pretty options, but there are still some cases where gold is the very best option.