I doubt he genuinely preferred either. He seems to favour whoever is less likely to reflect poorly on him at the time. I get the impression that the Lion is possibly one of the most vain and proud of the primarchs, as much as Fulgrim but in a different way.
He needs to be the 'winner', or be seen to be significant player, hence the dubious dealings with Perturabo, and Curze, the weird relationship with Luther, Terrans and Calibanites.
At least that's how I see it.