Platinum is more valuable, incredibly long wearing, and has a heavy weight to it that many enjoy. Unlike white gold, it doesn't need to be Rhodium plated. The downside of platinum is that it is very expensive. It will cost you double that of an 18kt white gold ring, and that's excluding any gemstone costs. However, it is a valuable and beautiful purchase.

Most white gold is Rhodium plated. You can find rings that combine platinum and natural white gold for a unique style and effect. White gold is an alloy of gold and white metals like silver and palladium. It used to be that nickel was traditionally used in the making of white gold, but this caused reactions to the metal in enough people to deter jewelers from using it in the white gold alloy. Just be sure to ask, especially if you're aware of a nickel sensitivity. It may be inconsequential since most new rings made of white gold are coated with Rhodium. The natural color of white gold is a light-gray color, and the Rhodium makes the white gold look more white. Rhodium is very hard, but it does wear away eventually. If you re-plate your ring every 12-18 months, however, you'll maintain it's appearance. The service is provided by most jewelers and cost effective. White gold is normally more expensive than yellow or rose gold.

Have you thought about a palladium ring? It's in the same family as platinum, 95% pure like platinum, it has a natural white luster, is naturally hypoallergenic, and it sells for the same price as a white gold ring. It won't turn yellow over time, either. However, it is less dense than platinum, and you won't get the nice heavy weight. But for look and wear, it's a nice alternative with value.

As a pure or nearly pure noble metal, platinum is much less likely to be chemically or allergically reactive to the body. So the chances of irritation are much less. As the main whitening agent in most white gold alloys is nickel, to which many people are at least slightly sensitive, this can be a rather important difference in itself. Platinum is denser and stronger than white golds. This means that settings can be made of thin, seemingly fragile constructions, and still have the needed strength and integrity to securely hold stones or jewelry together. Its density and strength combine as well to make it remarkably resistant to abrasion.

I prefer white gold because you can polish out the scratches an knicks, Platinum is a stronger material for diamond settings but can be pemanently scratched forever.