The best bridge would be one made by the most skilled and caring dentist and the finest lab technician.
It's the craftsmanship and not the materials. That being said, porcelain is used for esthetics. If you don't care about esthetics, an occlusal (biting) surface with metal cannot break, while porcelain can. In my mouth, i would go for implants, and the pfm bridge as the next best option.