Today, cast iron is considered a premium product - people pay extra for it. The main reason for this is that it is much quieter - you are less likely to hear the water running down the wall from the toilet, tub, or shower since the cast iron muffles it better than pvc. Depending on your local conditions, it can deteriorate, but usually lasts a very long time. PVC goes in quicker and isn't as likely to clog (not that a properly installed cast iron system clogs), mostly because the insides are smoother. If it is sloped properly, that doesn't happen much - the insides get a coating on it like a well-used frying pan. If stuff sits in it, it can be a problem, especially if your water is acidic (usually not a problem with public water supplies).