You can't go wrong with either Kershaw or Wainwright. I assume Matheny is going to pick his guy AW. And it won't be an injustice. Not like last year, where Matt Harvey started over Kershaw because it was at NY. Kershaw and Wainwright both have dominant stats. Kershaw will go in the All Star Game Tuesday with a 1.78 ERA, currently leading MLB. But Wainwright's is right behind it, at 1.79. Depending on how his next start goes on Saturday, he could be the leader. It is fair to point out that Wainwright has 4 more starts, and 35 more innings pitched than Kershaw, who missed 6 starts after opening the season in Australia. That makes Wainwright's ERA even more impressive.
They both currently sit at 11 wins, 1 behind the MLB lead. Kershaw at 11-2 in 14 starts, Wainwright at 11-4 in 18.
Wainwright leads MLB Pitchers in WAR (Wins above replacement, or basically how many more wins he's worth than the average player). He has an impressive 4.67 WAR
Kershaw is 6th at 3.7
Kershaw has 126 strikeouts, good for 11th in MLB, despite having at least 4 fewer starts than 28 out of the other 29 in the top 30 (Yu Darvish is the other, who has only 3 more starts than him)
Wainwright sits at 111 K's, good for 17th.
Kershaw's WHIP (the average of walks and hits allowed per inning pitch) leads MLB at 0.83. Wainwright is 5th at 0.92
Kershaw's Strikeout to walk ratio is an absurd MLB leading 9.69. With his 126 Ks, he only has 13 walks. Wainwright sits 20th at 4.11 (111/27)
Kershaw's Strikeouts per 9 innings also leads MLB at 11.77. Wainwright is 45th, at 7.63
So you can see, there are compelling stats for both players. I, no doubt submerged in Dodger bias, believe that Kershaw should get the nod, based off of this year alone. Throw on top of that his previous 3, its a no-brainer. I think that most other managers would also make that call. He pitched today (Thursday) so that means he will be on a regular 4 days rest come the Tuesday All Star Game. (Not that he would need all the rest, the starter will probably only go 2 innings, 3 max.)
Seeing as how the Cardinals Matheny is the NL manager, in charge of making the decision, I can easily see him rewarding his workhorse with the start. His last start is Saturday, which means he'll be pitching in the All Star game on what normally would probably be a bullpen day for him, meaning he can substitute his bullpen work for the 2 or 3 innings he'll pitch in the ASG. So starting wouldn't be an issue for him either.
Either way, its a good 1-2 punch for Matheny and the NL to have. Can't really go wrong with either.