I'm just old enough to have watched both players for the entirety of their careers (probably missed some of the really early Fedorov stuff). I think Fedorov was a special athlete, while Datsyuk is a special intellect/talent. But as differently as I think about those two guys, the end result for both of them was an immensely productive player who happened to be one of the best defensive players in the game - but maybe could have put up even higher offensive numbers if they treated the game the way an Ovechkin or Gaborik does.
In terms of Hall credentials, I think the two of them may end up being almost equal. Datsyuk's later years will end up being more productive, which balances out the fact that he wasn't as dynamic at his best as Fedorov was around 94/95. But it's telling that they could end up with somewhat equal track records, and Fedorov won 3 Cups as an elite player, while Datsyuk won us 1.5 (he was basically a 4th liner on that '02 team), and yet the people on this board would automatically retire #13, but largely take a pass on the #91.