Silver is a slightly better conductor than copper, something on the order of 5%. It is also a couple orders of magnitude more expensive. You can achieve the same result by using slightly larger copper wire at minimal difference in price. One can make a better argument for silver plated wire than solid silver wire because of skin effect, but it hardly matters for almost all applications. IIWY I would buy the cheapest 15' certified high speed cable I could find.
Flat cable is problematic. I doubt the ones I have seen measure well compared to typical round cables. That said, if you use it and don't get sparkles or worse, it is working as well as any cable will.