cast iron heavy,very durable, acrylic more popular but can scratch, so use the right cleaning products.

Here's some other info on tub material:

Cast Iron
Cast iron is a very durable material, usually covered with a porcelain enamel coating. A cast iron tub is arguably one of the most durable fixtures in a home, potentially giving reliable service for decades.

Cast iron tubs are usually of the alcove (surrounded by 3 walls) or free-standing variety. Their finish is durable and easily cleaned although you don't want to use aggressive scouring techniques or you could scratch the surface. Enamel that does eventually wear or chip away reveals the cast iron substrate which can rust. Finally, cast iron tubs are heavy and combined with the water weight, need good support structure.

Enamel On Steel
Enamel-on-steel tubs (or porcelain-on-steel as they're sometimes called) have a similar look to cast iron tubs except that they're lighter. Like cast iron, their surface is easily cleaned and maintained and their lighter weight means they're easier to muscle into a remodeled bathroom than a heavier cast iron tub. They're also susceptible to rusting should any of the enamel chip or get scratched, exposing the underlying steel.

Acrylic
Acrylic in simple terms is a form of plastic and is another material used to make bathtubs. It's upsides include it's high gloss, similar to the enameled look on cast iron and steel tubs, and it's much lighter weight than iron and steel. It can be scratched more easily but these scratches can also be repaired more easily than a porcelain enamel surface. Because acrylic is an easily-formed material, it's often used for whirlpool and air tubs which can be had in many different shapes and sizes.

Fiberglass Gelcoat (FRP)
Another material used to make tubs is fiberglass-reinforced plastic (FRP) that's covered in something called "gelcoat". You'll find some whirlpools and air tubs made from this material because it can be shaped easily. The gelcoat provides the smooth, glossy surface of the tub and offers an easily cleaned surface. It's a lot like acrylic but usually not as durable, as it can be scratched and/or cracked if it's struck hard enough.

Composite
Composite tub materials include American Standard's Americast products and Bootzcast from Bootz industries. These tubs are made with proprietary engineered composite materials that form the backing of the tub itself. The finished surface of the tubs are enameled. The intent with these kinds of tubs is to achieve the benefits of cast iron, like heat retention and a durable surface finish, without the weight associated with cast iron. These composite tubs can weight about 1/3 as much as cast iron, something to think about if you're replacing a 2nd story bathroom tub.

Some users report slight flex and/or creaking with composite-backed tubs though experience varies and is sometimes related to installation technique.

Hope this information clarifies not confuses the issue!

We are currently doing a remodel on two bathrooms and have the same question. Another question we have is: how easy are acrylic tubs to keep clean. There is a Kohler Mariposa tub that is extra deep that we are considering. You might want to take a look at that one. - Kelly

Thanks all for your info. I guess what I am really looking for is how to tell a good acrylic from a cheap one....it is the flex/creaking I am worried about. Hard to find cast iron soaker and would LOVE to avoid the 350 lb monster. Ideas or suggestions?

Your best bet is a name brand such as American Standard and Jacuzzi. The top end would be Kohler. On all 3, you get what you pay for.

Are you installing the tub yourself? I've done two person CI tubs on the third floor! Homeowner doesn't lift a finger. The Kohler Tea for Two is a good deep soaker in cast iron. Just put one in. Nothing beats cast iron. The tubs in three homes I've owned were/are cast iron. An antique claw foot and two two-sided tubs (no alcove and an ell shaped rod). Only one was original to the house and all three were over 80 years old. And in great condition....maybe not as glossy and shiny as a new one...but then at 80, who is?