Cast iron is great to look at and might even be an antique, it will last a very long time as it can be re-enamelled when it becomes scratched, chipped or shabby. It does however have many drawbacks. It is very heavy so will be difficult to install especially into bathrooms in upper floors. Bathroom floors on upper levels may need to be shored up to accept the extra weight. Making any remodel more expensive. Cast iron tubs are much more expensive that similar sized acrylic baths. With acrylic you can get the look of a cast iron tub, but at much cheaper prices. Acrylic tubs don't last forever and you will need to replace it ever 15 or so years. The biggest advantage of acrylic is its lightness, so they are easy to install even in upper floor bathrooms.