Acrylic is less expensive - which makes them popular. They can turn yellow-ish over time so you have to read the reviews and watch for a good warranty. The cast iron models cost quite a bit more, but don't turn yellow. You just have to find three or four really strong backs to get them into the house! If you are concerned with weight or yellowing, Victoria + Albert makes similar styles out of Volcanic Limestone. They are spendy, but they are like cast rock. UV rated, so you can use them anywhere without yellowing. Really neat designs too.

A couple of resources:

Victoria + Albert: http://vandabaths.com/us/americas/

Acrylic - Oceana: http://www.oceania-attitude.com/baths

Best of luck with your search!

Robert with BATHOLOGi LLC.

I have never seen an acrylic bath turn yellow mine is 10 yrs, old no yellow.