platinum s the metal of choice, although usually costs 4x what 14k runs. it is harder and wears into a beautiful patina. 18k has snob appeal but is somewhat soft. 14K yellow and white are the standards. I doubt anyone can really differentiate between any of the white metals (silver included) when you are wearing the piece.
I was under the impression that platinum is softer than gold, not harder. I've found that it scratches more easily than gold, and was advised that a platinum prong setting is safer because of platinum's softness - that if banged on something the prongs will bend rather than breaking off, thus securing the diamond more than gold would. Have I been misinformed?