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When talking about the MVP award, it's a common misconception to believe that it is simply awarded to who the best player is. In actuality, it goes to who is most valuable to his team, so there are many other determinants that need to be factored in.
One way we can determine who is more valuable to his team—Buster Posey or Andrew McCutchen—is take both of these players completely off of their team's roster and decide which team would suffer more. The player whose team would be worse off would be considered the MVP because they are the most valuable, which is the definition of the award after all.
Through this method, I would argue that Posey is more valuable to the Giants than McCutchen is to the Pirates, and here's why.
First of all, the Pittsburgh Pirates and San Francisco Giants are very similar teams. They both have below-average offenses, but very strong pitching staffs from the rotation to the bullpen.
Posey and McCutchen are also easily the best offensive threats on their respective teams, but the Pirates role players are having much more productive seasons than the Giants. The Pirates have four other players with double-digit home run totals, while the Giants only have one. The Pirates also have three other players with more than 50 RBI, while the Giants only have one.
As you can see, Posey, along with Melky Cabrera, have basically been the entire Giants offense, while McCutchen has been getting at least some support from his teammates.
Secondly, you have to take into account how valuable Posey is as a catcher. Not only would the Giants be losing Posey's bat if he wasn't on the team, but it would certainly affect the pitching staff—which is the bread and butter of the Giants.
So while losing either player would likely cripple both of these teams, I would give Posey the slight advantage in terms of how valuable he is to his respective team.