Is Stones any better than Dier at this point? They will both improve with age. Defending is about experience.
Stones has never been better than Dier ... even ignoring Dier's ability to play multiple positions
Dier was a key part of a solution that made Spurs much better defensively, Stones has moved to title winning side and has "helped" the defense look like league 1 quality.
In 3 years from now, Stones will be playing for Stoke/West Ham/whoever