To a large extent, your decision depends on what you want and your budgetary constraints.
Do you want to change to over to a hydromassage tub (spa type tub) while you have the other tub out? Make sure you will be able to run an electrical circuit to it if you do. If so, you’ll want a fiber glass, cultured marble, or acrylic tub. The type depends largely on the manufacturer of the tub you want to by.
Do you simply want the best, longest lasting tub? If so, then you’ll want a cast iron tub.
Do you want mid price range? Or cheap price range? If that is your goal, either the stamped metal or the two piece plastic / fiber glass will be your choice. All three types are available in various price ranges.