Dear 100 Hour Board,
I love my sea salt grinder, but I don't love the prices of the refills. Kosher salt is about the same size and I think I can get away with using it in the grinder instead. I don't salt my food very often, but which would be more "healthy": sea salt or kosher salt?
Well, actually, you can buy kosher sea salt. However, I'm going to assume your question refers to sea salt vs. regular kosher salt.
Anyway, Doctor Kanga and I had this long and extensive conversation about salts, and we came to the conclusion that salt is basically salt. Sea salt might have some extra minerals in it that table salt doesn't have, but you can always get those minerals through other foods and by taking a daily nutritional supplement, anyway.
Also, you may find this of some interest. According to New Italian Recipes, kosher salt refers to salt that doesn't contain magnesium carbonate (and has been blessed appropriately and so forth, of course). Dr. Kanga also found this article that talks about the properties of different salts, specifically crystal structure (course or fine).
Hope that helps you out. Personally, I'd just opt for whatever is cheapest.