What defined a prince? Nobility, courage, power, intelligence, compassion, the trust and loyalty of his people. There were more, there had to be more because Loki was a prince of Asgard and a true one at that. Not in the way Thor was, who embodied all those things and Sif could not imagine another king to match him. Thor would be a warrior king—may it be a long time in the coming Odin be merciful—but Asgard would continue to shine serene in her glory because Loki had the silver tongue that Thor lacked.
Honesty was another trait for a prince, and purity of heart. It made for rash decisions however, and as much as it was needed in the battlefield Sif had also learnt that court and politics required another sort of battle skill which she had never cared for. Loki never spoke out of turn, never raised his voice and had the singular ability to incite whichever response he wished from his audience. He was the only one Odin Alfather allowed to speak during diplomatic missions. He knew and appreciated the fact that Loki could at will deceive without lying.
A man with whom she fought side by side and had trusted her life to shouldn't have such an alien mind. He never faltered when he saved their lives, and Thor, Sif and the Warriors Three had at different occasions saved his—and there was a certain bond that came with that—but with Loki it always felt like he chose to take that step or throw that dagger that would spare them.
Which prince had the better traits for Asgard's throne Sif often wondered—and she wondered because she saw how it gnawed at Loki. She wouldn't have questioned it otherwise because Thor was the heir the eldest and the strongest. He was light and comfort and their leader. The only answer she could find was both.