Generally, preparing a tooth for a gold crown is the most conservative type of preparation. Tooth structure removal is minimal and, in some cases, healthy tooth structure can remain untouched. Although porcelain formulations have improved, porcelain is still a hard substance which can cause wear on opposing teeth over years of use. Gold will not do this. A well-done gold restoration can last many years.
Gold also tends to create a healthier environment for the gum tissue.
The major disadvantage, of course, is cosmetics. Most people do not consider gold to be an aesthetic substance. If it is in the back of your mouth, however, this factor may not play a large role in your decision. On the contrary, the major advantage of porcelain is aesthetics. Because there are quite a number of shades of porcelain, it is usually pretty easy to match the shades of your adjacent teeth.