BMW 120i 2009 Inner Rear Panel (located behind the safety bar) replace or repair

I got hit be someone at the traffic light. The rear parts of the car such as the: trunk lid, safety bar, lights and other small components will be completely replaced. Now, the inner real panel located behind the safety bar was only dented. It was not broken, cracked nor cut because the safety bar took the hit of the impact.
The garage and the insurance company gave me 2 options but I am not sure which option is optimal for my safety and my car's durability should it be involved in another rear accident (hope won't happen again!). Should I keep the original inner rear panel as it is (from the BMW Germany factory) although it was only dented or should I change it with the new one which will require melting, cutting the original part and welding the new part to the body of my car? I would like to mention that the new part will come from another car which I have no history of at the moment.
I have attached only one picture of my car with the denting repairs, since I cannot add more.
I hope this helps.

Thank you in anticipation and looking forward to the earliest answer.

Luiza