If you're voting Arrieta, you're basically saying it's because he has 20 wins (which Greinke could still get to). Greinke has a lower WHIP (0.85 to 0.90), lower ERA (1.65 to 1.88) and higher bWAR (8.7 to 7.4). Kershaw has a higher fWAR (7.8 to 6.2, though that's before tonight), strikes out way more 11.39 K/9 to 9.17), and has a better FIP (2.09 to 2.51). Both of the other two also walk fewer guys. Yeah, it's tough to make a obvious case unless you go wins and narrative. Arrieta does have a pretty sizeable advantage in ERA+ though. But yeah, every argument for Arrieta over Greinke is an argument for Kershaw over Arrieta. Maybe try to convince people that his dominant stretch in the second half was when it matters more or something, but that's a road I usually don't go down. I guess for traditional voters, Arrieta might be able to edge Kershaw in IP and CG as well, but they are really close and there won't be any significant advantage there.