Gold purity confuses a lot of people, so let's clear it up simply: the karat number tells you how much pure gold is actually in the piece. 18kt gold is 75% pure gold. 9kt gold is 37.5% pure gold, just above the legal minimum to even be called "gold" in many markets.
That difference in purity changes everything about how the metal behaves. 18kt has a richer, warmer color because there's simply more gold in the alloy, and it's generally kinder to sensitive skin since it contains less of the other metals that can cause irritation. 9kt, with more alloy metal mixed in, is noticeably harder and more scratch-resistant, and it costs meaningfully less since you're paying for less actual gold.
In my years going between suppliers and retail counters, I've seen 9kt do very well in everyday-wear pieces, especially in markets where it's the traditional standard, while 18kt tends to be the choice for bridal and heirloom pieces meant to be passed down. Neither is "better" gold, they're built for different jobs and different budgets.
My honest advice: if you're buying a piece you'll wear daily and want it to shrug off scratches, 9kt is a smart, practical choice. If you want that deep, unmistakable gold color and you or the wearer has sensitive skin, spend the extra for 18kt. Ask your jeweler to show you both side by side under the same light. The difference is easy to see once someone points it out.