Matthew, I recently read a book by Jim Bolen called "Goof Proof Health" and it is mainly a book that is written on a layman's level spreading the knowledge and work of Dr. Batmanghelidj and the Water(&Salt) Cure.
In Bolen's book he talks about how salt(on the tongue) with water, helps get the water into your body's cells and hydrate you, where sodium and potassium are in perfect balance. In the chapter for Cancer, he recommends soaking in a bath water of Sea Salt , because dehydration is a large culprit of cancer and also many other diseases. It is supposed to be highly therapeutic, even for pain.
I believe that Epsom Salt would be for the purpose of soley absorbing Magnesium, so I think it has it's own merits, is a different therapies, but probably very complementary.
All unrefined Sea Salt that is solar dried and hand harvested is good! The wet and mushy looking and feeling Sea Salt s, like Celtic, I personally think are better, because they have a bit of a lower amount of sodium in there, and a higher amount of magnesium and potassium.