I personally think that Ross will be a better player than DeMar, but ultimately this is Casey's decision.
I think it comes down to whether or not DeMar is a better listener than Ross. I don't think it will come down to skill level. If DeMar does what Casey wants him to do, and doesn't cry about it, then he's going to get the minutes. The starting job is his until his ego gets in the way, or until he goes down with an injury. That's my guess.