what he's saying is that if you try to modify the back to cut out the tail pipe pieces in urethane, you can do it, but the filler will seep and you'll see it like a week later. so fiberglass it is for the rear. now his reason for the fiberglass was the paint will adhere to it better than urethane and it takes a lot more finagling with urethane. he said yes urethane is more durable but its more likely that you'll have issues with it cracking if you bump anything. "sure the bumper is more flexible but the paint doesn't flex with it" as he said.
Well I dont think the urethane will be a problem cause if it was, Ford would have made fiberglass instead of urethane. Also if you hit anything with the fiberglass bumpers kiss it goodbye and I dont mean just the paint, you will destroy the glass.. If I were you I would do fiberglass for the rear and urethane for the front.
