Porcelain on cast iron is a very hard surface, that, if you don't use an abrasive cleaner can look like new for decades. The porcelain is similar to glass. The CI doesn't deflect and stress the material and the plumbing fittings. Installed properly, you can minimize the flexing on an acrylic tub, but the surface is still plastic and will scratch. If you drop something hard on a CI tub, you can take a chip out of the finish, you may not do that on an acrylic, but could scratch it. Both can be touched up, but if needed, a properly done repair job on an acrylic tub could be invisible. It's harder to do on a CI tub. A CI tub should outlast an acrylic with the same use by a long shot and still look good.