I'm planning a bathroom remodel and am looking at slipper baths. I want water to remain warm and need one to fit my smaller space. The acrylic is about $800 cheaper.
cast iron takes longer to heat up but stays hot a lot longer.
So if you want to start filing it up then get in right way while it finish filling and get out soon then acrylic is fine.
With cast iron you need to start full hot to heat up the tub them less hot to get the right temp. and let it at least 1/2 full before getting in. Otherwise the cast iron will feel cold.
It will stay hot for a good long time! 2x longer than acrylic in my experience.
cast iron hands down.
grew up with one.
have acrylic now because it came with the house.
everything karl says about if you want to soak it won't be in an acrylic is true.
i have purchased an older cast iron tub that i'm going to be setting up in my garden for my soaking pleasure. i have access to hot water via an outdoor shower and i will be purchasing a camp stove burner to go under it as well. that's how much i don't like my acrylic tub for soaking.
i would also say that if you do enjoy being in a tub, divide $800 by the number of times you will use it over your residency in your current home. it could be surprisingly inexpensive in that context.
One thing to remember is that a cast iron tub is much heavier than an acrylic. Something to remember if the bathtub is going upstairs. Who is going to carry it up there and making sure your floor/ceiling can support the weight. My friend had a problem with hers causing the floor/ceiling to sag. So whoever is doing your install make sure he knows what he is doing.