Partly, I will give you my opinion....this is not a scientific study :) . Partly from a real estate value perspective.
The look of a cultured market tub will give your home a more updated look therefore showing better and perhaps selling better in the future. I do think from personal experience that they cool the water off quicker. I would be more concerned about the size of the water heater. Make sure you have plenty of hot water, then it won't matter....enjoy that cultured marble tub!!
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My best gess would be hot water would cool much faster in a fiberglass tub due to the thickness of the material. However I can also see a cultured marble tub cooling just a quickly depending on the temperature of the tub prior to pooring a bath. It would take long for the hot water to bring the marble to temp. and in the mean time the marble would pull the heat right out of the tub. I would honestly guess the fiberglass tub would keep the water hotter longer ... This is a tough one, perhaps myth busters time, ha
Keller Williams Realty
Real Estate Consultant