Yes, white gold is harder than platinum. But another thing to consider, especially for an eternity band, is the longevity of the metals- white gold wears much faster. To polish white gold, metal is removed (and it needs to be polished every time before it is rhodium plated) To polish platinum, it is more of a "reshaping" process, repositioning the metal that gets pushed around during wear. My main concern on an eternity band would be that in ten years there will probably not be very much metal left holding in those diamonds as it has all worn away, whereas the platinum will still be there holding in the diamonds, especially on the bottom part. Also, under stress, platinum is more likely to "give or bend" a bit, whereas white gold is more brittle and likely to hold its shape until it breaks or until a prong is completely sheared off. No metal is bulletproof, but given the facts I would choose platinum.