A cast iron tub will be the most durable. They're heavy and can be difficult to move and emplace without help or a little experience. Some people love Americast tubs, some hate them. Acrylic is available in more shapes and is lighter. It probably has the easiest finish to scratch (that being said, it can last a very long time and look good), and probably requires more care in setting to ensure it doesn't squeak or flex, so it can crack. Because it is easier to shape, you'll find more shape choices in an acrylic tub (which is what I put in). if you set it properly, it will be as solid as the others. Use the search function, there's a lot of discussion on these. If you like the look of the CI tub, it is the most durable.