Are porcelain crowns as strong as regular teeth?

No: While porcelain and now Zirconium crowns are very strong, life like and function as natural teeth, your own teeth are best! Of course, if it's a necessary need, these replacements for your missing or damaged natural teeth are awesome!

Stronger: zirconia = strongest with almost 1200mpa (+-) normal tooth = 305mpa (+-)

Of course: porcelain- Zirconia is harder than tooth enamel. However, nothing can take the place of the teeth you were born with.

Natural teeth vs....: That's an interesting question. If strong means least likely to have problems long term, then a natural tooth without restorations of any kind is the strongest. There are numerous types of "porcelain" and the strongest being zirconia which is stronger than enamel. However, a crowned tooth can get marginal leakage, may not be as esthetic and may dislodge or break with insufficient tooth preparation