What decides the choice
Start with the metal, because it decides both what you pay for and how the piece wears. Gold is sold by carat, a measure of purity out of twenty-four: 9 carat is a little over a third gold and hard-wearing, 18 carat is three quarters gold and softer, which is why it scratches more readily on a ring worn every day. Sterling silver is 92.5 per cent silver and tarnishes. Plated pieces carry a thin layer of gold or rhodium over a base metal, and that layer wears through first where the piece rubs against skin or clothing.
If you have sensitive skin, the base metal matters more than the finish. Nickel is the most common cause of a skin reaction to jewellery, so a description that names the base metal is more useful than one that only names the plating. Treat the word hypoallergenic as a description rather than a standard, and look for the metal itself: higher-carat gold, platinum and titanium are the usual choices for sensitive skin.
Then look at the parts that fail. Clasps, jump rings and the claws that hold a stone are where most jewellery is lost, far more often than the chain or the stone itself. A lobster clasp holds more securely than a spring ring, and a stone held by six claws keeps its grip if one of them bends. Ask whether the seller resizes rings and repairs clasps, because a piece that cannot be serviced has a shorter life than its metal suggests.