You can read the first piece comparing the catchers to get an idea about the metric being used here. To give a short bullet point about ZiPS - it will hopefully give us a relatively accurate look at how a player will perform in the future based on their performance in the past. It weighs four years of data, other metrics and uses age regression.
In the catcher comparison, New York's Brian McCann was projected to have better stats over Boston's Christian Vazquez. But he also costs almost $5M more per WAR.
Now we look at first base. Boston and New York have two of the older first basemen in the league. Which doesn't matter if they can give you the production you need. But neither Mike Napoli nor Mark Teixeira played in more than 125 games last year. They both had an OPS under .800 as well. So how does 2015 look?
Boston: Mike Napoli
2015: 115 G | 95 H | 54 RBI | .241 BA | .353 OBP | .441 SLG | .794 OPS | 2.4 WAR | Cost per WAR: $6,666,666.67 | Age: 33
New York: Mark Teixeira
2015: 90 G | 77 H | 65 RBI | .236 BA | .326 OBP | .433 SLG | .759 OPS | 1.5 WAR | Cost per WAR: $15,416,666.67 | Age: 34
Ouch. Teixeira's number are actually a little better than his 2014 performance, but that may be because he is projected to play 33 fewer games. Napoli's numbers are down a little across the board except for his slugging and OPS, which are both slightly higher.
Neither player is particularly cost-effective at their position, although Napoli is much closer to that ideal. Teixeira's insanely high cost is even more stark when compared to other high-money first basemen. Miguel Cabrera costs Detroit a little over $5.5M per WAR. Jose Abreu is costing the White Sox only $3,333,415 per WAR. Even Albert Pujols, who has one of the most insane contracts in the majors (he'll make $30M at age 41!), is costing the Angels $6.67M per WAR. Granted, these costs will go up over time and Abreu is six years younger than Teixeira. But this does give you an idea of what Teixeira is costing the Yankees by being their first baseman. And that contract has one more year on it. Napoli's contract runs out this year.
The real kicker here is that Napoli is providing Boston with almost 1 full win more than Teixeira is giving New York. With a cost that is 43% cheaper than what Teixeira costs New York. That hurts.
But, before Boston fans start patting themselves on the back, here is one more line. Let's call him Player X
2015: 152 G | 153 H | 94 RBI | .269 BA | .358 OBP | .493 SLG | .851 OPS | 4.2 WAR | Cost per WAR: $1,190,476.19 | Age: 25
Okay, let's not call him Player X anymore. That, Boston fans, is the projection for Anthony Rizzo. The young first baseman the Red Sox traded to the Padres for Adrian Gonzalez.
So while Teixeira may have a slightly better stat line, Napoli is providing more WAR at a lower cost. To me, that gives Boston the edge at first base over the Yankees.