PVC, or polyvinyl-chloride, is a coating that's durable, resists abrasion well, and sheds certain chemicals like diesel fuel. It's lightweight and relatively inexpensive. The way PVC seams weld together makes them the most waterproof. PVC tends to collect condensation and perspiration between your skin and the inside of the garment, and in cold weather, it may become brittle. The quality of PVC gear varies widely.
Polyurethane is a rubber polymer coating.
It's lighter and more flexible than PVC, but it's also less durable, and is usually applied inside the nylon shell. It's more breathable than PVC, but making it more breathable on the inside means reducing its ability to repel water from the outside. Better quality polyurethane gear is carefully bonded in many thin layers to prevent water absorption that can result in eventual delamination. Neoprene is the most flexible and durable of the coatings. It's very popular in England and other areas where boating is not just wet, but cold, and boaters are willing to suffer the weight and the cost for guaranteed waterproof gear that lasts.