Porcelain tubs are relatively inexpensive, but they do not conduct heat well. Cast iron tubs are very heavy, but they maintain the temperature of bathwater longer.
One type of bathtub is a formed-steel option with an enamel coating -- this is called a porcelain tub. It is the least expensive material for bathtubs, making it common. The porcelain surface makes these tubs easy to clean.
However, the steel conducts heat, which cools the bathwater. Another disadvantage of porcelain bathtubs is that the enamel is prone to chipping. The exposed steel can then rust. Porcelain tubs also aren't the lightest option, so contractors may need to make adjustments for second-floor bathrooms.
Cast iron tubs are also coated in enamel, though the layer is thicker and more durable. This means the enamel coating doesn't chip as easily as with porcelain. The cast iron itself is also more durable, making it resistant to impact. The cast iron does pull heat from the bathwater, but it eventually conducts the heat back. This function keeps the water warmer overall.
The main disadvantage of cast iron tubs is that they weigh between 350 to 500 pounds. That much weight makes them difficult to install. Likewise, contractors may need to reinforce the floor. While the enamel coating on cast iron doesn't chip as easily as with steel, it's still possible. Exposed cast iron can rust.