Most baseball people are saying that Posey deserves the award, and I don't have much of a problem with that. However, I think McCutchen had a season that should seriously be considered for the award. Here's my case.
The name of the award is Most Valuable Player, not most outstanding player or best player. I think that a lot of the time, voters vote for the best player instead of the most valuable player, there's a huge difference there. Keeping that in mind, here's some statistics:
The below picture shows McCutchen and Posey's individual seasons. I also took out McCutchen and Posey's statistics from their teams final season lines, just to see how much better these players were than the rest of their teams. Here's what it looks like (click to enlarge):
You can see the differentials there. McCutchen raised his team's batting average by nearly 100 points, their on-base percentage by 265 points, and their slugging percentage by 24 points. Posey's differentials don't compare with McCutchen's. I understand that this isn't the fairest way to look at it, because we are basically giving McCutchen credit for being on a really bad team. However, doesn't that prove his value to the team? He took a team that hit .262/.314/.381 without him and got them 79 wins. Posey played on a team with Melky Cabrera who hit .346/.390/.516 until mid August when he was suspended, Pablo Sandoval who hit a very solid .283/.342/.447 in a full season, and a pitching staff who put the National League's 5th best ERA. Posey was certainly very valuable to his team, but I think it's safe to say that the Giants still would have been in contention without him.
You certainly cannot say the same for the Pirates and McCutchen. The only offensive players that you could even argue for having a good offensive season were Pedro Alvarez (.244/.317/.467 with 30 HR), Neil Walker (.280/.342/.426) and Garrett Jones (.274/.317/.516 with 27 HR). The Pirates were average at best on the mound.
Let's take a look at the correlation between these players performances and how their teams did throughout the season. Here's a chart of McCutchen's performance by month, followed by a chart of the Pirates win percentage by month, and then the same for Posey.
Posey:
Both players had some really great months and some not so great months. McCutchen's worst was August when he hit .252/.347/.346, the Pirates win percentage dipped heavily with that at a .393 winning clip. He also struggled in September, although some of the power came back, Cutch hit .254/.361/.500 and the Pirates won just 25% of their games. In Cutch's best month, July, Cutch hit .446/.510/.739 and the Pirates won 65% of their games. It's a pretty direct correlation. When Cutch was good, the Pirates won, when he wasn't good, the Pirates didn't. Obviously there was more to it than that, but the point remains true.
Posey has some correlation as well, but not nearly as striking. The Giants catcher had his worst month in May when he hit .253/.311/.363, however the Giants still won 52% of their games. Posey had a stellar second half of the season. In July he hit .381/.448/.595 but the Giants had their worst winning month of the year winning just half of their games. Posey kept going and hit .371/.482/.652 in August while his team won 62% of the games and the Giants were even better in September, winning 70% of the time while Posey hit .364/.415/.607. The Giants were still a .500 team while Posey wasn't doing much at the plate.
Again, this a one season sample, and you can't take all that much from what I just said. Just because Posey had bad month and the Giants still won half of their games doesn't mean that they would have been a .500 team with him hitting .250 all season long. That said, this part of the study certainly favors McCutchen for MVP.
A lot of attention goes to Wins Above Replacement when you're looking at the MVP picture. Posey does have the edge in that category (7.2 to 7.0 on baseball-reference and 8.0 to 7.4 on FanGraphs). A lot of that has to do with Posey playing catcher, which automatically helps his WAR number. Defensively, Posey was superior to McCutchen, who was a below average center fielder. All-in-all I don't think it's all that fair to just look at WAR. That number compares these players to a league-average replacement player, and has nothing specifically to do with the teams they play on.
The word "valuable" really is what makes me think that McCutchen should win the award. I personally don't really agree with the award being about value, because that eliminates some players right off the bat. Nobody from a really terrible team could ever be considered for the award, which isn't fair in my eyes. Every player strives to be the best player in the league, not the most valuable. Why not make the most prestigious award in the sport available to all the players, and not just the one on good teams? That's a question for another day. If we're talking straight value, and we are, the edge goes to Cutch in my eyes.