If your floor/joists can support one, cast iron is great. No flexing (tub floor), retains heat better, and more durable (won't crack). We installed two cast iron tubs (Toto 1525's) in two bathrooms, and a cast iron shower receptor/pan (Kohler) in the guest bathroom. Very happy we stuck to guns on those.

We went with AS americast. It's porcelain , more durable than acrylic but not as sturdy as cast iron.

Another vote for cast iron--we have Kohler Villager in kids' baths in current house as well as last house. Don't have to worry as much about ruining the finish if the wrong type of cleaner is used. Ditto the other cast iron attributes listed by cat_mom above.