Xehanort is an entirely different case, he willingly gave up his heart to darkness similarly to how Scar did. However what contradicts this is that the other apprentices willingly gave up their hearts but only produced puny shadow heartless. Scar necessarily doesn't have to have a nobody, and even if he did I doubt it would effect the story in anyway in future games. The fact still remains that Scar gave up his heart to darkness willingly, but how he differs from Xehanort is still a mystery.
But then arises a question about Clayton. Both he and his counter parts Stealth Sneak were heartless, or were they actually one in the same only split in two halves? Think about it for a moment, remember Xehanort's heartless? He had his Guardian which acted as his means of attacking Sora, and in a way seemed to be the true side of Xehanort..the darkest side to him. You can say the same of Clayton in a sense; after all he did ride on top of Stealth Sneak like a horse jockey. That and Clayton acted as an avatar since you had to defeat him only. It's a bit hard to explain.
