If you believe that gold can serve as money/currency, then you may ask yourself what other metals could serve a similar purpose. You might also consider whether there would be any benefit to having multiple metals serve as money.
If someone wanted to sell a product, would it be easier for them to quote a price in ounces of gold as well as silver, and maybe even platinum? I would think it would be easier just to quote the price in gold. With the advent of debit cards, there is no need to carry around gold ounces to complete the transaction, so I don't see any benefit to having price products in other commodity metals.
With that in mind I believe other non-gold commodity metals should not receive a premium price based on their use as money. This premium should only be granted to gold. Let's face it - gold is the standard, the ultimate money. All other metals are just commodities, and should be priced based on their mining cost.
That is a pretty heady premium.
Impala Platinum (OTCQX:IMPUY) is one of the world's largest producers of platinum. On pg.12 here, they list their platinum cost-per-ounce at $1,461 (10,994 ZAR at 7.521 ZAR per USD). On pg. 6, they list their price-received-per-ounce platinum at $1673.
Based solely on a comparison of mining costs and spot prices, silver is hopelessly over-priced when compared to platinum. I think there are only two responses to this: sell silver and buy platinum, or sell platinum miners and buy silver miners.
Both metals have large ETFs that hold the physical metals: SLV for silver, and PPLT for platinum. Many investors prefer Sprott's physical silver closed-end fund PSLV over SLV. Currently there is no similar fund for platinum, but that is changing soon. Sprott is planning a combined platinum/palladium closed-end fund. This may provide a positive catalyst for platinum in the near future - similar to what happened to silver with the creation of PSLV.
If you follow any of the "thought" leaders in precious metals, you will probably notice that they make almost no distinction between gold and silver. I believe that the reason for this is that gold is expensive. The average person is going to have trouble coming up with the $1800 it costs to buy an ounce of gold. At $40 for a one-ounce American silver eagle, a lot more investors can be turned on to the idea of owning precious metals. Don't buy into the hype. Don't ignore the fundamentals of market capitalism - producers flood into areas with increased profit margins. The bigger margins for silver miners indicate that there will inevitably be greater relative production of silver than of platinum.
Only gold is the ultimate money. All other precious metals can and must be compared on the basis of mining cost, spot price and supply/demand. These metrics indicate platinum as a much better opportunity than silver.