I have removed a rotted copper drain which was under a main floor slab. Should I replace it with PVC or replace the copper piping?

Replace with PVC. You'll not regret it. Good luck, Tom

Thanks you for the quick response Tom, now I have one more question. There is a fitting which connected the copper pipe to the cast iron drain. I heated it but it is very suborn and won't move with my pipe wrench. I can see treads by it's really on there. How can I remove this or can I leave it on and use some sort of quick connect for the PVC?