I don't disagree with your reasoning why the Red Sox are the better team. If I had to make a prediction, I would say the Red Sox win the division and the Yankees win the wild card. I would be thrilled with that outcome.
The reason I say they represent two games is that lets say, for example, the Red Sox were only 7-1 this year against the Yankees. The Yankees would actually be winning the division by a game if that were the case. When you look back on it, those games represent a two game swing. The main reason the Red Sox are ahead of the Yankees is because of those eight games. The Yankees have been significantly better against everyone else than the Red Sox have. If the Yankees are able to play .500 ball against the Red Sox the rest of the season, they really have a good chance to win the division, especially if both team continue to play the same against everyone else.
However, like I said before, I think the Red Sox are a better team right now than the Yankees because of starting pitcher.