I doubt they'll attempt to plastic weld a cracked bumper. They usually replace the bumper cover (that's pretty standard) If they're telling you they will only repair that by plastic welding - I WOULD be on the phone with a lawyer - that's not acceptable or garanteed safe. Also don't be afraid to talk to a lawyer - usually first visit is free and if you win the other guys' insurance WILL HAVE TO PAY. And since he backed into you, "I think you're holding 4 aces vs a pair of twos"
As far as me, I'm an audio engineer -live shows, concerts,etc.. but I am responsible for 13+ audio crews worldwide (w/partners) and all the nightmares that go with it.