What is better--gold or porcelein crown?

Equal. They both are effective replacements for teeth. Porcelain tends to be a little more expensive (although that might change with the increase in the cost of gold) and is used for teeth that might show in the front of the mouth. Gold, which is easier to fabricate, is used more towards the rear of the mouth.
Gold crown. This answer is based on longevity. The best marginal fit, wear, and longevity of the restoration is a full gold crown. However, most patients prefer the natural looking crown. A compromise is the porcelain fused to gold crown.
It depends. Gold has been used for a long time in dentistry & is very similar in hardness to natural tooth structure. Porcelain is very hard & glass-like and could potentially abrade the opposing natural tooth more so than a gold crown. I generally tell my patients that they should have similar material opposing each other. I would never recommend a gold crown for a tooth in the front though. Gold is also $$$.
Depends on situation. In almost all western nations, a "tooth colored" tooth would be preferred in the esthetic zone, a gold crown strictly in terms of function would work well any where, especially om the molar teeth. I would expect that if you polled American dentists, most would prefer a gold crown to restore a posterior molar, although new "Bruxier" crowns are becoming very popular and show promise to be as strong.