sea salt vs Kosher salt
The first difference between the two is that Kosher salt rarely has iodine in it. Sea salts are often unrefined and contain many more minerals, including iodine, from the sea. Iodine is good for most people and the U.S. mandated it be put in refined table salt. Some people don't want iodine or anything added to their salt.
But, that doesn't really answer your question yet, huh.
You may answer your question about quantity to use in your cooking by comparing the size of the salt crystal. Some people say that Kosher salt is stronger tasting than regular table salt. And, others say it is just the size of the crystal.
You may also answer your question by doing a taste test is a cup of water. Put equal amount (let's say 1/2 tsp) of salt in 4 oz of water and drink it. (What you've done here, too, is a new craze of healthful internal cleansing - drinking water with sea salt in it instead of regular water. But, I digress.
For a good site with all kinds of descriptions of salt sources/kinds, see http://www.saltworks.us/salt_info/si_...
As, poster "whatsfordinner" said, sea salts taste different. However, one doesn't have to use the sea salt AFTER cooking ONLY. It can be used in cooking, boiling, crusting, whatever you want.
Sea salt used to be much more expensive in days past. So, many said sprinkle on after cooking, instead of use sparingly because its so dang expensive. A container of regular ol' iodized table salt can cost 49 cents and sea salt or Kosher a little more. Some sea salts can still be super expensive. Trader Joe's has sea salt that is super inexpensive.
The difference I state on the two of your choice is iodine. Kosher salt is a refined salt with few, if any additives. Kosher salt usually doesn't have iodine in it. Sea salt is often not refined, and most contain iodine. Some say iodine is necessary in your diet for a healthy thyroid.
My understanding is that sea salt is used as an accent after cooking while kosher salt is used during cooking to develop the flavors in the food. I love sea salt sprinkled on a slice of really good fresh crusty bread spread with a good (unsalted) butter, but I love, love, love it sprinkled on fresh foie gras!