Cast iron is enamel over actual iron, whereas fireclay is a dense china. Fireclay is stronger than the vitreous china they make bathroom fixtures out of, but it can chip and crack. If you over-tighten the drain flange, especially around a garbage disposer, it can crack right across. Chips can show through to the raw clay color underneath the glaze. You have to be gentle with it, not just toss heavy things at it. Additionally, there are variations in fireclay sinks. You have to have your sink on site to template it, and there can be as much as a quarter inch variation from the nominal size, or even from one side to the other (some brands are probably more consistent (i.e., don't sell the ones that are more off size). OTOH, fireclay feels terrific. It has a density and tactile sensation that is unlike any other.
The current Kohler cast iron, which is a very high fire glass enamel, can scratch if you do just the wrong thing to it, but it won't crack or stain, and the company has a warranty. Other current cast iron sinks are of variable quality and generally not as good as Kohler. There are 100 year old porcelain on cast iron sinks that look brand new. The current Kohler surface, unlike the old porcelain, never needs bleaching.
For durability, Kohler cast iron wins. For deliciousness, fireclay wins. Which you choose is a personal decision.
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