I can't really compare Barkley to Sterling as they are such different prospects. In my books, I don't think there are many young players in the world who have the tools that Barkley does. His combination of strength, skill and pace is a rare one and although the comparison is, at this stage, ludicrous, the player he most reminds me of in style is Zidane. If he ends up half as good as Zidane I'll be chuffed, because I don't think I've seen a better player in my lifetime.
Sterling is a fantastic player as well, who also is nowhere near his potential and should have a terrific career. But in my opinion, Barkley is something else entirely. Martinez often says how unique Barkley is, and he is dead right. The reason I loved Zidane so much was because there was nobody like him, and Barkley is shaping up in a similar way.