I will now say that it is Days of Future Past by a large span. Yes, its "peripheral" characters (i.e. anyone not named Logan, Xavier, Erik, or Raven/Mystique) were underdeveloped, but that's to be expected from Singer. I was surprised how much that ended up applying to Whedon's Avengers too who all felt as broadly drawn as a Saturday morning cartoon for me in AoU. In comparison, McAvoy's Xavier goes on a very heartfelt and, dare I even say it, quietly thoughtful journey that grounds the movie in much pathos and weight, especially when dealing with Mystique whose relationship with Charles also gives the movie a youthful rebellious streak--more post-Vietnam rebellious than the generic teenage variety.
But also, the action is again much better. It does not have the whiz bang spectacle, but the future Sentinels were scary. They were incredibly menacing while the Ultron-bots looked like they were made out of paper with the way the Avengers joyfully cut through them.
So yes, overall I cannot think from my perspective of a single thing that the Avengers movie did better in this comparison.