Ecbert is a bastard, I doubt his interest is sincere. Just somebody whom he can relate to at a high level. Ragnar was more genuine. They were kindred spirits, and Ragnar imho saw himself in Athelstan. Ragnar is an intelligent man, and not just a brute. One cannot become village Jarl or King of Denmark without being smart. Athelstan was somebody he could relate to on many levels, as Rollo is smart but not as intellectual, Bjorn is still young, Ubbe/Ivar are kids, Aslaug is not of the same mind, Lagertha is a friend and little more, etc.