Kwahi is a better all-around player and more consistent. He had three great games stretch vs LeBron in 2014 finals, didn't destroy LeBron in these three games, but canceled him out completely. And whenever he cancels out opposing team's #1 option, Spurs win automatically. He is a better core player to build around.
Klay has higher one-game ceiling. 37 in a quarter, 11 3-pointers in the toughest game of the Steve Kerr era where he was the most dominant player on the court. He is comparable to Ginobili on the classic GDP big three teams. When you are desperate, he steps up and delivers a game far above his average games and gets you over the finish line. You can not build around him, but with his lack of ego, the perfect player next to a MVP.
Seriously Splash Brothers is the most amazing backcourt in NBA history. Can you name Jordan/Kobe's point guards? How about Stockton/Nash/Kidd/Payton's shooting guards? All blue collar grunts. Klay would have been first team all-NBA if shooting guard still has its own slot like center does, he and Curry's games are perfect for each other.