Question: as to longevity and durability, is cast iron or copper superior?
How long, if any, is the galvanized pipe under the concrete slab?
Do I need to replace the pipe under the concrete slab?

I have a house in Northern California built in 1956 with 2 stories and 2 bathrooms on the second story. Outside temperatures range from 30-70' F. My DWV, which is original, is a mixture of cast iron (CI) and galvanized steel. Two years ago, my kitchen sink/gutter area of the sewer backed up with leaves and was successfully snaked out. 5 years ago, the master bathroom sink and tub were clogged and successfully snaked. Last year, I had an earthquake retrofit done, which involved replacement of a portion of the kitchen sink vertical drain and the gutter drain was re-routed outside. The vertical galvanized pipe was rusted on the outside AND inside, but flow was possible through both pipes. Both pipes (sink and gutter) were clearly galvanized when they entered the floor/slab/concrete floor. I saw a rusted pipe that looked galvanized inside the slab. I plan to live in this house indefinitely. One contractor recommended copper, as the total cost would be the same for iron, and did not recommend changing the under slab pipe. Another contractor recommended CI, and replacement of the pipe under the slab which was galvanized.