Acrylic and Cast Iron Tubs Compared in Practice
Heat
Cast iron with its porcelain enamel takes heat out of the water at first and then holds it, so a bath stays warm noticeably longer. Acrylic does the opposite: it warms instantly and loses heat faster, though a unit with a foam backed shell narrows the gap considerably. For a household that actually soaks, this is the deciding property.
Weight and What Carries It
An empty cast iron tub is a serious weight before any water or occupant is added. On a ground floor over a slab that is irrelevant. On a suspended floor it is worth a look at the joists and the span, particularly in an older house where a previous alteration may have notched something.
Support Under an Acrylic Base
An acrylic tub is only as solid as what is under it. Set on a proper mortar bed or its supplied cradle it feels firm underfoot for decades. Set on nothing in particular it flexes, the flex works the seal at the wall joint, and water eventually gets behind the surround. Most complaints about acrylic are really complaints about installation.
Damage and Repair
Acrylic scratches and can be polished out. It also cracks if something heavy is dropped, and a crack in the base is difficult to fix invisibly. Enamel chips when struck and the exposed iron then rusts, which needs attending to promptly but is a small repair.
Choosing on the Room
An upstairs bathroom in a mid century house with a narrow stair is an argument for acrylic on logistics alone. A ground floor bathroom where the tub is used properly and often is an argument for iron. Both are correct answers to different questions.