Well I'm no Predator either but I would guess that the Predator would take the skull of prey that actually put up a fight you can see that in the movies to back that up. But I link this to Predator: Concrete Jungle. Think you collect the skulls os Seritur Houngan, the Dead Man Triads, Bruno Borgia, Hunter Borgia, Monster Squad, ect. But El Hongo, he was important he was a leader but still you only skin him. Why because he didn't put up a fight. All the others that I mention plus the Bad Bloods they all put up a fight leading to their death.
In the movies he took Billy's skull because he took a sword, cut himself and I would guess but up a fight. King Willy he had those two swords and it seemed like him and City Hunter had a fight before his skull was collected as well. Being a leader could have had something to deal with it but it depends how challenging the prey was to kill. Again in Predator Concrete Jungle despite El Hongo was an important leader but he just ran away which made him not an honorable kill. Plus in the movies the Predator's seemed to be hunting their prey which they would have collected their skulls because Harrigan and Dutch definately were honorable trophies.