And this is why I prefer to do as much of my own work to my car including oil changes.
It makes sense to let them attempt to repair the bumper the easier way for them for the sake of them not labeling you as a "pain in the butt" customer. I know mistakes happen and that's just life, but keep in mind its your car and they're the ones who screwed up your bumper. So to pick what's convenient for them for the sake of not being marked as a trouble customer in the future is just absurd to me. Unfortunately, this is how society works now-a-days; even though they screwed up you'd probably be remembered for the "trouble" you caused them despite the fact that they created said "trouble" themselves. Pick what will make you the happiest, and not make your choice based on what they might think of you in the future.