Despite Lake Erie's salt mines in my backyard, I use good ocean salt. Celtic is supposed to be the best, and is slightly gray. Sea salt is said to be healthier than plain ol' sodium chloride in that it has some magnesium, calcium, potassium, and other electrolytes. Sea salt is said to be worse than table salt because it lacks iodine. (Potassium-iodide has been added to NaCl in North America for 80 years; we have controlled Cretinism, but some say we're seeing more hypothyroidism as a result.)
I don't know if the differences are worthwhile, but ocean salt tastes better to me. But then again I'm a food snob.