Hermès continues to be the most reliable luxury stock, even if Richemont is catching up. Are Cartier Love bracelets about to replace the Birkin bag as the product that drives the best shareholder returns in the luxury-goods industry?

For years, Hermès was the go-to for anyone who wanted a company that could grow sales through thick and thin, thanks to seemingly bottomless demand for the Birkin handbag. But lately, the brand has been unusually weak. Meanwhile, sales at Cartier's owner Richemont have been booming for around 18 months.

Hermès still has an edge. Its Birkin handbags continue to sell at a premium to retail prices in the second hand market. This suggests that investors' worries that the brand's top product is losing its appeal are overblown.

Cartier's business is booming. The brand is a hit with Gen Z. Its Love bracelets are all over social-media feeds, and Taylor Swift wore Cartier jewels at her recent wedding to pro football star Travis Kelce.