Watching the two play, there is no doubt in my mind, that right now, Sanheim is the better player than Fleury.
I've posted many, many times that Fleury ahead of Sanheim was the right choice at the draft last year. Given the information at that point, where Sanheim had only played 1 full season to Fleury's two, and had only played a half season at a top level against Fleury's full season, I fully support Fleury being drafted ahead of Sanheim.
That being said, Fleury has plateaued, or even regressed this season while Sanheim has shown astronomical growth. It's difficult to compare an offensive defenceman to a defencesive one, but give credit to Sanheim to winning the WHL defenceman scoring title. Sanheim's name is in the conversation for WHL defenceman of the year, Fleury's is not.
Further, watching the two, Sanheim drives the play whenever he is on the ice. Fleury does not. Again, they are different stylistically, but you notice Sanheim every time he is on the ice and you don't always notice Fleury. I am still high on Fleury and still think he'll be an NHL defenceman, but right now Sanheim is the better prospect.