Shalin Shah I think Booker is more of a threat to Ronnie Hillman, who may not make the final roster, right now. If CJ falters, then I could see Booker taking over a lot of snaps. And I think he’ll have a role in the offense, especially if Hillman isn’t kept.
But traditionally, Kubiak has been a big believer in a bell cow back. He was the offensive coordinator for Mike Shanahan during the Terrell Davis years, and even after Davis, Shanahan usually had one main running back. As the coach of the Houston Texans, he was the one who found Arian Foster and coached Foster’s biggest seasons (as both a rusher and receiver). And in his lone year in Baltimore, look at what Kubiak did with Justin Forsett.
So I think his preference would be for CJ to be THE Guy, with Booker and/or Hillman as complimentary pieces.