Stones is ahead of where Cahill was at 22, but the question is whether Stones will actually go on and develop into a better player. Cahill developed loads at Bolton and then at Chelsea. Stones clearly isn't improving at all at City, he looks more shaky and error prone than when I saw him play for Everton as he is getting badly exposed by a tactically inept manager (when it comes to defending). Evidently, the bizarre tactics and pairings Stones has had to endure (e.g. playing with Kolarov at the back) are not helping his game.
IMO there's no real evidence to assert, with any certainty, that Stones will be better than Cahill as things stand.