I heard that cast iron will keep the water warmer for longer. To me, that would be enough of a selling point by itself.

I prefer cast iron for cleaning and looks but I believe they are more expensive.

JDez-
I'm sceptical about cast iron keeping the water hotter longer. In fact, from a thermodynamic standpoint, cast iron has a higher mass and heat capacity, which means that it will cool the water initially more than the acrylic because it will suck more heat out of the water and it will be more efficient at radiating the heat to the surrouding air. Acrylic is a poorer conductor of heat, so it will slow heat loss from the water in the tub compared to cast iron.

I'm sure you're right kudzu. I had read that on GW somewhere and I believe everything I read.

If you go cast iron, order the tub early so it can be placed in the bathroom when the house is being framed up. Have the GC build a plywood box, a strong one, to cover it. They are a real problem when people wait until the place is framed and you try to carry them up stairs. They are heavy. I absolutely prefer CI to acrylic.

As previously pointed out you do have issues with the mass of the tub. If you maintain 70* in the house the tub will reach that 70 degrees and sit there until you fill it with hot water. The tub will then absorb whatever heat it needs to get up to the new equilibrium temp of around 105*, your max bath temp. It takes some energy to do this and you have to be prepared to replace that energy, or you end up taking a lot of short baths, which is REALLY annoying when you get a soaking tub.

When I was in the business I used to run a 4' loop of hot water baseboard element below/behind the tub. It would turn the CI tub into a radiant heater and provide rely nice bathroom temps. In fact, it could get hot in the bathroom. You could do this if you are using a boiler for your space heating. Coupled with a radiant floor, you have a nice package.

In our almost finished re-model I installed a Victoria Albert Asia model tub. It is a cast limestone product and I had one in my previous home as well. It has much the appearance of CI without the weight and mass. They are worth looking at.

To feed the tub I use a tankless water heater. I have mine set up with the bath fill control that allows me to set the bath temp I want (104*) and the vol I need (35 gal) to fill the tub. I then turn on hot only and when the 35 gal of 104* water is delivered to the tub it shuts off the water and beeps to tell me my bath is ready. Very cool, imho! The other advantage to tankless in this application is that I can peak the tub for as long a soak as I want.

Good luck with your new home.

Where will the tub go? I'd choose cast iron, but not if it is upstairs.