I'd still give Stephenson the nod as the top guy due to his stuff and his age. Fun fact on Stephenson: 375/378 batters he has faced this season are older than he is.
As for the control, it is probably unfair to take away certain games and look at a clipped version of stats. But it is at least worth noting that he had 2 terrible games where he gave up 7 walks in each. Take away those 7 innings, 14 walks and he's right below his career average of about 3.5 walks per nine innings.
Half of a season isn't a super small sample size, but it is interesting to me that he only had those 2 games where control was a major issue. He had 2 others where he gave up 4 walks but actually pitched pretty well. In every other start he had 1-3 walks, which is right where you'd expect him to be.
Major control problem or just 2 bad nights where he was off?