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!!
We sell homes in Southlake, Westlake, Grapevine, Colleyville, Keller and for that matter most of North TEXAS.........let us know if we can assist in selling the home you are moving from. Be sure and check out our web site!