One thing to consider is how you will actually use precious metals for payment, barter, etc. If you go to a dealer (coin shop, some jewelers, etc.), they will accept coins without much hassle, since they are known and trusted to be what they say they are. For example, a 1 oz U.S. silver Eagle is going to be 1 oz of .999 silver. If you bring in a bar of 1 oz. silver, and it has the assay markings on it from a known company, it might be accepted as such. If you bring in a 1 oz bar with no markings, then it might get a bit more complicated, not something you need in a TSHTF scenario. They might have to assay it before they can accept it, because they don't know if it is .999 silver or something less.
With that in mind, buillon coins from country mints, such as U.S Eagles, British Sovereigns, etc. command a higher purchase price. Not only are they a stated metal content, but a beautiful coin as well, with more value than just metal. Buillion bars with assay markings would be next, followed by unmarked bars.