A couple more things to consider. Performance against good teams.
Career numbers against teams with below .500 records
Sale 30-12, 2.53 ERA, 0.955 WHIP
Kershaw 66-26, 2.48 ERA, 1.051 WHIP
Career numbers against teams with .500 or + records
Sale 36-28, 3.01 ERA, 1.108 WHIP
Kershaw 54-31, 2.32 ERA, 0.991 WHIP
Kershaw's performance improves against better teams.
And as was alluded to earlier, Kershaw gets better as the game wears on.
Since 2011, starting pitchers (min 100 starts) ERA against batters in their 1st plate appearance,
MLB rank.
1. Clayton Kershaw, 1.72 ERA
9. Chris Sale, 2.72 ERA
Since 2011, starting pitchers (min 100 starts) ERA against batters in their 2nd plate appearance,
MLB rank.
1. Clayton Kershaw, 2.21 ERA
4. Chris Sale, 2.49 ERA
The second time through the lineup, Sale's ERA is only 0.28 higher than Kershaw's. But, it's in that third time through the lineup where Kershaw separates himself from Sale.
Since 2011, starting pitchers (min 100 starts) ERA against batters in their 3rd plate appearance,
MLB rank.
1. Clayton Kershaw, 2.44 ERA
25. Chris Sale, 3.59 ERA
Again, Sale is very good. But Kershaw is the best in the game, and by a comfortable margin.