This is a really tough race right now, with close to ten guys you can make a reasonable case for. The NL's top pitchers should all be in the conversation (more on them in a minute), and among position players, the choice really comes down to how you value various components of the game. Buster Posey has put up great numbers, while mostly playing catcher, baseball's most demanding position. Yadier Molina hasn't hit quite as well as Posey, but has still been very good at the plate, and is the best defender at the position, maybe at any position. Carlos Gomez was already been a great fielder and base runner, but he's suddenly become a very good hitter as well, and leads the league in bWAR. Paul Goldschmidt and Carlos Gonzalez are each in the discussion on the NL's best hitters, and do well in other facets of the game too. Andrew McCutchen, also playing great I'm going with David Wright though. He's provided as much value on offense as anyone, and aside from the two catchers, he plays the most challenging defensive position, and plays it well.
My choice: David Wright
Others who'd be on my ballot: Paul Goldschmidt, Carlos Gomez, Carlos Gonzalez, Matt Harvey, Clayton Kershaw, Andrew McCutchen, Yadier Molina, Buster Posey, Adam Wainwright
Who actually wins it at season's end: Andrew McCutchen. I think he'll keep posting solid numbers and if the Pirates make the playoffs for the first time in 21 years, he'll have a powerful narrative. Guys like Wright and Gomez have no shot, because their teams are too bad, and while that shouldn't matter to voters, it will.
Like the MVP race, this one is really too close to call right now. Unlike the MVP race, there are really only three candidates (with apologies to Cliff Lee, who's having another fine year). Matt Harvey just started the All-Star Game, and has exploded on the scene this season. The other two have been great pitchers for a while now. Here are some pertinent figures for each:
Matt Harvey: 10.18 K/9, 1.94 BB/9, 0.48 HR/9, 153 ERA+, 2.17 FIP
Clayton Kershaw: 8.61 K/9, 2.61 BB/9, 0.43 HR/9, 181 ERA+, 2.52 FIP
Adam Wainwright: 7.98 K/9, 0.98 BB/9, 0.37 HR/9, 149 ERA+, 2.23 FIP
Kershaw holds the lead in bWAR, while Wainwright is tops in fWAR. I don't know how you mount a strong argument for any one of them over the two others, they're all very reasonable choices. I tend to fall more to the ERA+ side of things than the FIP side, when rating players' past performance, and Kershaw has the edge there, so I'm backing him.
My choice: Clayton Kershaw
Others who'd be on my ballot: Matt Harvey, Adam Wainwright
Who for actually wins it at season's end: Adam Wainwright. I have to think Matt Harvey's season will get stopped early due to him hitting some sort of innings limit, which will make it hard for him to win. Between the other two, Wainwright has two factors working to his advantage, I think. The lesser is that he hasn't won the award before, and I think some voters like to see new winners. The bigger factor is that he currently holds a 12-8 lead on Kershaw in wins, which shouldn't matter to voters, but clearly do. Wainwright is on pace to win 20 or 21, while Kershaw will have a hard time getting to more than 17 or so.
Rookie of the Year
Probably the biggest story in baseball over the last month and a half has been Yasiel Puig, who didn't debut until June 3rd, but has already hit 8 home runs while putting up a line of .391/.422/.616 playing half his games in a pitchers' park. If he's anywhere near that kind of player in the second half, he'll deserve to run away with this award, and maybe draw some attention down-ballot in the MVP race. I'm not yet ready to put him above a couple pitchers who've been performing really well all season though: Jose Fernandez and Shelby Miller. Like the trio I mentioned for the Cy Young, these two are awfully hard to separate, they've even thrown an identical number of innings thus far. Miller looks just a little better when you look at metrics like FIP, because his strikeout and walk numbers are just a bit better than Fernandez's. Fernandez has allowed fewer runs though, and there's a noticeable difference between them when looking at a metric like ERA+. Just as that was the deciding factor in my Cy Young vote, it makes the difference here.
My choice: Jose Fernandez
Others who'd be on my ballot: Shelby Miller, Yasiel Puig
Who for actually wins it at season's end: Yasiel Puig. Fernandez is just 20 years old, and won't be allowed to pitch all the way to the season, likely being shutdown around the start of September. In a race with multiple strong candidates, that could sink him. Puig is going to slow down, a .472 BABIP isn't sustainable for anyone and his strikeout-to-walk ratio is somewhat alarming. Still, I think he'll maintain strong enough production to end up with 20+ HR and his rate stats will still be very impressive. Miller is likely to regress downwards a bit as well, and I think Puig wins this battle.