Yesterday in interleague, the AL outplayed the NL 11 games to 2. Bringing their season record to 97-56. That's a .634 winning percentage.
One way to think about this: the Mets are easily the best team in the NL. But if we were to give the average AL team (say, Seattle, or Minnesota) an NL-only schedule, they'd be up on the Mets by about 2 games.
Another way to think about this: the Royals have a winning record against the NL.
(We've discussed this here before, but it goes without saying that because of this, Bronson Arroyo would not being doing anywhere nearly as well with the Red Sox as he's currently doing with the Reds...but don't tell him that...)