I'll be honest, I'm on Sejuani's side. This is mainly because I think that Ashe is basically backing out of the sort of "Grand Freljord History" of winner takes all and such. It's a continent made of ice. It's not really meant to be united and made a utopia, like say Demacia or some of the other Pre-Rune War era cities that were paragons of civilization. It's an ice cube with inhabitants.
tl;dr Ashe is a person trying to make things good that aren't supposed to be, and Sejuani is a mighty and rightful ruler.