I think the reason Quintana and Nibali aren't considered at the level of Froome and Contador is because they have not outright beaten them on the grand stage. Both won a GT last year, but neither faced Froome or Contador. I also think those last two have never faced each other on an equal level apart from the start of the Dauphin? last year and the Ruta del Sol this year. In the Vuelta 2012, Froome was tired from the Tour, in 2013 Contador wasn't up to his level, their first crossing of swords in 2014 was at Catalunya, where Froome was recovering from illness and was behind on racing. At the Dauphin?, they both started out well, but then Froome crashed. In the Vuelta, Froome's form was clearly behind Contador's, because he was unable to do any of his usual accellerations.
Apart from that, I think Sky has more superdomestiques for Froome in the mountains than fit in the team, while Tinkoff-Saxo has not for Contador.
Froome, Porte, Nieve, K?nig, Roche, Thomas, Kennaugh, Poels, Kiryienka and Siutsou for Sky
Contador, Kreuziger, Majka, Basso, Rogers, Kiserlovski and Hern?ndez for Tinkoff-Saxo