A cast iron tub is heavy, and you better be sure your floor can support it with the full weight of a tub filled with water and a person inside! If all this tub is for is a seldom used bathroom, then a steel tub is fine. The only real downside to these tubs is that if they chip, they will rust.
An acrylic tub on the other hand is light, easy to keep clean, and generally comes in alot more styles/finishes. They are admittedly flimsy feeling when you step into them, but if you install them properly on a bed of morar, they become as solid as any of the other options, and if you want something fancy, you can get a whirlpool option. Although I generally feel that having a jetted tub is more trouble than they are worth.