I don't pretend to love the Avengers, it's not my preferred style, but I have great respect for what it did. 
Avengers gave character arcs and roles to Stark, Widow, Hulk, Thor, and Cap; it brought them together which is its plot achievement, and it shifted public attitude within the world in a convincing manner to being more favourable to the heroes. Those are natural plot and character developments.
X-Men DoFP starts with a plot that skips a lot of steps not just from First Class but from X3: The Last Stand / The Wolverine as well. Whether or not that's a problem is up to you, but I personally argue that the movie is less good than it could have been due to skipping steps. They also skip steps within the movie. Mystique's character arc is to stop being an assassin and to go back to being good. Why does she do so? Because Xavier nags her throughout the movie and at the climax scene, once the plot demands it, is where she finally caves in and becomes good again. That's problematic writing. 
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Though, FWIW, I think that First Class is better than both