Fireclay is more durable and is made of a specific type of clay found in a few areas of the world. The clay is blasted with incredibly high heat (about 2200 degrees!). It's less likely to chip and lasts longer than porcelain. It is non-porous so it doesn't hold odors or stains.
Porcelain is generally cheaper than fireclay. Porcelain sinks are often made from cast iron which has been coated with a porcelain glaze. The issue here is that the porcelain coating runs the risk of being chipped.