It's got nothing to do with stats. If anything, playing alongside McFadden actually hurts Jones. Call it the Hambrick Syndrome where a player looks better behind a good RB, than actually being 'the man' himself.
And your comment about Mendenhall saying he could not beat out an undrafted FA is a bit puzzling. As a backup, Mendenhall had 78 carries for 640 yards (8.2 avg). To me, it seems like a case of just not getting a chance. But if we were to hold players by the same standard, we can only assume that Romo was not as good as Bledsoe, Colombo not as good as Petitti, Barber not as good as Jones, etc.,
As I clearly stated before, I want the player who is the best. Not the one that is simply different and gives the best change of pace. I want a player who can step in for an injured player and have no drop off. If Mendenhall was the higher rated player as the Cowboys FO has said, then take him. Rely on what you have scouted, seen and put down on paper about these prospects rather than simply who will give us a change of pace.
If Jones was the higher rated of the two, then I would have no problem. But it just bugs me we simply are not taking the best available player at a position we draft at.