Cast iron is the cadillac of tubs. Very durable and long lasting. It is possible to chip it , but it does not chip easily, like porcelain on steel does. The Vikrelll is also a quality tub. If cared for properly, it would be quite long lasting, but comparing A to B, cast iron probably has the longer life expectancy.
Any type of fiberglas or acrylic surround will have to be caulked in the seams. In fact, even a marble or tile surround will have some seams to caulk. There is no secret to mold prevention......ventilation. If you do not have a good exhaust fan, any tub/shower will be prone to mildew. You must use a quality mildew-inhibitive caulk, and clean FREQUENTLY, and immediately at the first sign of mold, with Moldex. If mold is allowed to remain and grow on caulk, it is incurable. And if not properly treated when you scrape out the caulk for redo, the mold will reappear.