What is the best tub material?

I am getting ready to remodel my 1950s bathroom and trying to decide what is the best type of tub (material-wise) to buy. I'm on a budget and looking only for a simple rectangular tub (5' x up to 3') to soak in that won't chip and will look good for a long time.

I like the tub I have, but it is 50 years old a bit worn and pink.

Question:

Is it safe to assume my existing tub is cast iron? I don't know how to determine what material it is made of.

I'd like to get an acrylic since it's lighter and easier to install, but I"ve been told it won't hold heat and may chip more easily. True?

How about a porcelain steel tub?

Any advice (tell me what kind to buy, please) would be greatly appreciated.

FOrget the porcelain over steel. Acrylic can chip if you drop something on it, but so can some of the other things, including cast iron. At that age, it probably is cast iron. On any tub, a little insulation suffed in around it can help with the heat. If you use the search in the bar above, you'll get lots of hits where this question was asked.