The Jones aren't going to be bring in any quarterback who will pose a threat to Dak. So Cam and Winston are out.
Besides, I don't think Cam has developed his passing game anywhere close to where Dak is, and that's not saying much.
Cam thought his size would be enough to will wins simply because he was a bigger and faster quarterback. But he found out that NFL defenses are the great equalizer. I don't think he can consistently run a traditional passing game with the quarterback staying in the pocket.
As for Winston, he's too immature to lead the Cowboys.
I'd stay away from both of them simply because neither will want to be back-ups to Dak. I'm sure both think they're better.