Here are your choices:
Plastic, either fiberglass or acrylic, offers the greatest design flexibility because it can be molded into many shapes. It's warm to the touch and insulates well, too, so water doesn't cool as fast as in enameled steel or cast iron tubs. Plastic is also the lightest tub, weighing in between 60 and 70 pounds. Although it doesn't chip easily, abrasive cleaners will damage the surface.
Enameled steel, formed steel with a porcelain enamel coating, is the least expensive tub. But the material brings drawbacks: Steel conducts heat, meaning tub water cools quickly; the surface is prone to chipping; and it weighs about twice as much as plastic.
Cast iron tubs, like steel, are coated with enamel. However, they don't chip as easily as steel because the enamel coating is thicker than on steel tubs, and cast iron is more durable and resistant to impacts. At first, a cast iron tub will pull heat from water, but once it heats up, it will keep water warm for a long time. Cast iron's main drawback is its weight, 350 to 500 pounds, which may complicate upstairs installations.
Cast polymer tubs traditionally replicate the look of marble, granite, or onyx, and are now available in a range of solid colors. Cast polymer costs a little more than acrylic; however, its surface doesn't stand up as well. With time, the gel-coat finish on cast polymer tubs can become brittle and expose the material underneath, leading to cracks. If cast polymer provides the look you want, shop for brands certified by the Cultured Marble Institute or approved by the Institute of Associated Plumbing and Mechanical Officials.
Proprietary composites, relatively new entrants to the bathtub marketplace, include American Standard's Americast and Briggs' UltraTuff materials. Heavy-gauge steel, porcelain enamel, and resins combine to create a tub that offers all the benefits of cast iron with half the weight.
Enameled steel tubs start at $100 and can go as high as $300. Cast iron versions are more moderately priced at $82 to $170. The least expensive option "plastic" start at less than $100 and can sell for $130.
Of the plastic tubs, acrylic is a better buy. It's more expensive than fiberglass, but more durable, more resistant to staining and fading, and easier to repair and clean.