What is the difference between the 4 inch model 620 and 686? If I wanted a new medium frame S&W, why would I pick one over the other. They are about the same price and weight (except the 620 is a little lighter because it doesn't have a full barrel lug. I think I read that the 620 was a modern model 66, which I thought was a stainless model 19. Is the 686 a more durable gun?