Which one is better, ductile iron pipes and mild steel pipes as pressurized pipelines?

For watermain pipe size less than 600mm, ductile iron is normally used because internal welding for steel pipes below 600mm is difficult to be carried out. Moreover, it requires only simple jointing details which allows for a faster rate of construction. For watermain pipe size above 600mm, steel pipes are recommended because steel pipes are lighter than ductile iron pipes for the same material strength and therefore the cost of steel pipes is normally less than that of ductile iron pipes. In addition, in areas of difficult access the use of lighter mild steel pipes has an advantage over ductile iron pipes for easy handling.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.