I've been curious about this, and my suspicions were confirmed. According to
FiveThirtyEight's interactive Week 14 playoff implication chart, it's better for the Cowboys' playoff chances if the Eagles beat the Seahawks (an Eagles win improves our chances by about 5%, and a Seahawks win decreases our chances by about 5%). Of course, this is because their model has the Wild Card as the far more likely route for the Cowboys to make the playoffs. FiveThirtyEight has us at just 12% to win the division, but 49% to make the playoffs overall. Our chances of winning the division do go up by 6% if Seattle wins and down by 5% (to a paltry 7%) if the Eagles win.
Not surprisingly, the Cowboys/Bears game has a huge swing on our overall playoff chances, +12% if we win, -20% if we lose.
They also added a cool interactive tool that lets you pick the Cowboys (or any team), and you get a chart showing which teams you should root for this weekend for the Cowboys to increase their playoff chances. Apparently, the result we should be rooting for the most is Tampa over Detroit (at +8%), which makes sense. After that, we should want Philly over Seattle (+4.6%), then Oakland over SF (yeah right, +4.3%).
This all kinda geeky and no doubt an inexact science, but I have fun with it. Feel free to check it out:
http://fivethirtyeight.com/features/nfl-week-14-playoff-implications-the-chargers-have-a-pulse/