Out of the QBs that the Cowboys could possibly get, Mannion is the most ready to play, having been in a pro style offense for four years, and Grayson also has experience in a pro set. Petty hasn't used a playbook before, and has most of his reads and throws dictated by the spread formations that Baylor used. If it were me, of the three, I would go with Grayson - he's the best combo of current readiness and upside of the three. Petty may have the most upside overall, but he has the largest ability to bust, IMO.
I would look at Shane Carden late in the draft.