Tackling stats for super 14 from foxsports (as I know how you like to use stats): Conrad Smith 72/87 (83%)
Jaques Fourie 72/94 (77%)
This suggests that there is not a large amount of difference in tackling ability (and suggests Fourie does indeed miss the odd tackle).
I think you are way off on your assessment of Smith - he' by far and away the best 13 in NZ, and is only getting better. In fact in his last All Black match (versus France last year) he played in my opinion his best ever game for the AB's. He is a different sort of player than either Fourie and O'Driscoll - his strengths are organising the backline, and making the right decisions both in attack and defense, while he does have the odd moment of brilliance (e.g the try versus South Africa last year). Fourie is probably stronger and faster than Smith, but is nowhere near as good as Smith at setting up his outsides (which, in my opinion, is one of the primary roles of a center). I'd say O'Driscoll, Fourie, and Smith are clearly the best three centers in the world - on form I too would probably rate Smith ahead of O'Driscoll (and we will see how they go head to head in the weekend).