Quickley is one of the better defensive guards in the league. He's a total menace, and you can really see this by the way he rotates around the perimeter. Covers so much ground and does all the little things. He is a massive upgrade on that end. Offensively, while Sexton is a good+effiicient scorer, he doesn't necessarily make his teammates better and he's often a ball stopper. Still a good offense player in his own right, but Quickley plays within an offense better IMO.
Both guys were really good as starters, but Quickley was insane. You have to balance that with his suspect playoff performances, but he definitely deserves the higher price tag between the two. I think Quickley is the new Lowry, they may not be flashy or look the part of a star player, but they contribute sooooo much to winning. It's not an accident that year after year he's a +/- god. While the basic statistics may not be close, it is not remotely close by the advanced metrics.