White gold is a mixture of yellow gold and white metals that give it a white appearance.  Platinum is a naturally white metal.   Both precious metals are perfectly suited for jewellery: non tarnishing, non-allergenic, and capable of being intricately worked.

The price difference is subject to market fluctuation; however, platinum is heavier, and there is a higher labour component in the finishing costs.

White gold is normally finished with rhodium plating.  When this wears off, white gold may show a feint tinge of yellow.  Polishing and replating will restore the finish to original condition.