Is ring-spun cotton worth paying more for?

Usually, yes, within reason. Ring-spun yarn is spun to be smoother and stronger than cheaper open-end yarn, so a ring-spun tee generally feels better and resists pilling and blowouts longer — which supports its higher price on a cost-per-wear basis. "100% cotton" only tells you the fiber; ring-spun tells you what was done with it, and it is a common, honest line between a $12 blank and a $30 staple. The caveat: ring-spun is table stakes at the quality end, so it is a floor, not a guarantee. Weight, construction, and fit still decide whether the specific tee is a good buy.