The Giants Oline is a big weakness, and will really help the Cowboys' defense look good in the opening game. With Dallas actually having a good DT rotation and Sean Lee, they will shut down their running game. It's going to be a pass happy game for Manning, especially if Dallas' offense gets off to a hot start. That's going to lead to the usual Eli interceptions.
Also can't discount how the two teams look in preseason. Dallas has looked focused and sharp, while New York has looked the exact opposite. I'm not saying that means the Giants will have a bad season, but we saw in 2014 how a sloppy preseason can carry over into the start of the season. Dallas looked terrible week 1 against the 49ers, and didn't look that great against an overmatched Titans in week 2.